If 4-ohms basically equals twice the power output for your amp over 8-ohms, and 2x power = 3db. Why bother with a 4-ohm cabinet? Is some greater benefit to be had than volume alone, perhaps in increased headroom?

A 4-ohm cabinet doesn't automatically guarentee twice the output from every amp. You choose a 4-ohm cabinet if it suits your amp, and your playing conditions. There are countless combinations of circumstatnce that might make this an optimal choice. You might want a minimal setup, and need a 4 ohm cabinet to get sufficient output from your amp. You might want a 2-ohm load, and two 4-ohm cabs is less unweildly than four 8-ohm cabs. And so on.

While this is true, it is a stat that is blown out of proportion in my experience. There's a huge difference in perceived volume between an amp that puts out 200 watts vs. an amp that puts out 400 watts. Maybe this is because if you push a 200 watt amp at 200 watts, it is working ALOT harder than a 400 watt amp at 200 watts. I don't really know the reason, but thats just one of those stats that people quote that may be true in theory, but on a gig that power makes all the difference.

Example. Player has tube amp that has a 2 or 4 ohm output tap. It's not in the best interests of the amp to use a 8 ohm cab on it's own. Player buys 1 4x10@4ohms cab since he does not need the benefit of two cabs and his amp is been used saftely.

I get 200w out of my amp at 8 ohms, I get 350w at 4 ohms. 350w is much louder, so that is good. It's a pretty basic concept.

What i was going to say Or what about people who want to run different speaker types? let me tell you, my peavey firebass with just the 4x10 or just the 2x15, is loud, now with both speaker cabs going, it doubles the wattage (almost) and fairly increases the surface area Doubling the wattage will give you ~ 3dB more yes . . . and so will doubling the speaker area, so you'll get about ~6dB more, which is quite a bit! And if you dont believe me about the speaker area bit, think of say an ampeg SVT, it is 300 watts on each impedance tap, now, with one 8x10, its loud, with two 8x10's, it'll still be 300 watts, but it will be fairly louder! So, you could say its alot to do with people getting the most out of them, i know you were meaning with using a single 4 ohm cab, but hey, wheres the fun with only one

I want to use the biggest and baddest amp just like any proper bassplayer. If I had my way I'd use 2 vintage SVT's + berg NV610 + NV215. Unfortunatley the people that pay me have different ideas

I guess what I'm driving at is: 2 identical cabs, all else being equal, the only difference is the impedence, will I get any perceptible real-world benefit from choosing the 4-ohm cab?

In a SS amp, yes, in a tube amp that has taps for both impedances, no What you have to take into account is possibility of adding more cabs and maximising headroom from your amp with SS anyway, and mainly just for matching impedances with tube Audiophiles will sometimes say there are differences sonically, but, there is nothing noticable, certainly nothing in a band situation or any other situation really!

Decibel's aren't a linear measure they're exponential so a 3db increase is much more than you think. And for reference, if you were to increase the output by another 3db, by more speaker surface area or whatever, then you the difference between the 6db and the 3db increase would be a lot more than your would think. It's physics, that's what it is...

not necessarily. ya see, wattage is nothign compared to surface area of the speakers. give me 200 watts and 6 10s, and 400 watts with 2 10s, and i know who's gunna win

who? From your post it's obvious that you think that the 610 wil be louder with half the wattage, but why?

Double the wattage is a 3 db gain Double the speakers is a 3 db gain Therefore 200watts with 6x10 is greater than 400watts with 2x10. This is presuming that other factors are the same.

I have two 4 ohm cabs. Given my druthers, I would have choosen 8 ohms. Why did I pick 4? Because they where only available in 4 and they sound really really good While my amp supports 2 ohms, and most power amps can handle 4 ohms per side, 2 ohms does limit your amp choices. And you never get twice the power going from 8 to 4 ohms. As mentioned previously, tube amps tend to provide the same power to both impedances since they drive an output transformer that converts to the proper impedance. And I am not sure that I buy the "you get more headroom" concept. At 4 ohms you are working the amp harder. The whole point of headroom is to give the amp breathing room so it isn't working as hard. And, as johndanielmoore mentioned, some tube amps do not provide an 8 ohm tap. If you want one cab, you have to get 4 ohms.

Worrying about an amp's performance with 4 ohm loads is unneccesary. Punishing subwoofer jobs are handled all the time at 4 ohms, often bridged - which is somewhat like running two 2-ohm loads. If you have an amp that can't deal with 4 ohms but was spec'd as being able to, you have a problem. Otherwise, why worry - be happy ; }

But I think you're misunderstanding headroom. You don't get more headroom by getting less power. More headroom is exactly what you get when you have more power available. That's why you want more power. If you have 400 W available as opposed to say 225, then your amp has to work less hard to get an equivalent volume. This is assuming that a 4 ohm load is not so close to the amp's limits that it stresses it unduly. Most current bass amps of which I'm aware can handle 4 ohms loads without too much stress, the exception perhaps being some stereo amps that are bridging into 4 ohms, which is basically a 2 ohm load on each side--something that can be stressful for some amps. But most good current mono amps don't find a 4 ohm load too stressful. By the logic you suggest, wouldn't a 16 ohm load would be even better than an 8 ohm load? But you can't really make the argument that having around 1/4 the power available can possibly give you more headroom.

Even if an amp is designed to handle a 4 ohm load, it still has to work harder. The current demands go up. And it is harder to provide current than voltage, especially when 3 large tube amps are plugged into the same 15A circuit Let's look at it the other way.... why can no amps (or at least none that I know of) handle a 1 ohm load? The current draw would be too great. So I'm not saying a well designed amp will have problems at 4 ohms, just that it has to work harder. If you let it work easier, it will handle peak demands better. And being able to handle peaks is my definition of headroom. Feel free to argue that I am wrong. I'll have to think about that; there has to be a point of diminishing return. And I think you would have more than 1/4 the power. It dosen't halve every time. Some caveats here: I am no expert, so take this all with a truckload of salt. It is all IMHO. And I *can* be convinced I am wrong. Also, I am not talking borderline cases here. If you are running the amp flat out and need every watt you can get, get a 4 ohm cab. Although I would argue you need a bigger amp. And just last saturday I ran the amp at 2 ohms for a 1 1/2 hour set with no problems. So I am not trying to scare anybody away from 4 ohms.

The essential point remains, though: you don't get more headroom by having *less* power. It doesn't work that way under standard operating conditions. You get headroom by having *more* power, by definition. Sure, a stable and reliable 200 W is better than a 400 W that you can't get without causing your amp to shut down every 2 minutes, but as a rule, that's not really the choice you're facing when you have a decently designed and made amp and you're considering whether to use a 4 ohm or 8 ohm cab. You're generally choosing between a stable 8 ohms and a stable 4 ohms. And as far as not working the amp as hard goes, you are working it less hard when you use a smaller percentage of its max power. If you need 100 W, you are working the amp less hard if it has 400 W available than if it has 200 available.

A big +1 on this. Virtually all high quality, modern SS amps are designed to run 'optimally' into a 4 ohm load from my understanding. By optimally, I mean putting out the maximum power without generating problematic heat or degrading the signal by adding more THD or whatever. The way I look at it, impedance represents the cab's impeding power. Why buy a head that is designed to produce 550 clean and cool watts at 4ohms and then 'limit' that head's output to much less than that by running into an 8ohm cab. Also, as pointed out in various posts above... an 800 to 1200 watt difference 8ohm vs. 4ohm might not be that noticable if you aren't working the amp hard at its 800 watt level. However a 200 watt to 400 watt 8ohm to 4ohm difference could be huge if you are running the 200 watts full bore.