I am considering a larger rig, probably a head, 2x10 cab, and 1x15 cab. When looking at, say a 1x15, I may find an 8ohm cab that can handle 200W, and one that is 4ohms but can handle 400W. Are these cabs comprable in volume? Or does the increased power consumption mean higher sound levels?

i play guitar and know more about guitar amps, but this should hold true for bass amps, too: ohms are current resistance. less ohms = more output watts are measures of output. more watts = more output 400watt@4 ohms would be louder. but.... most cabs and heads (and PAs, etc) and different ohm input/output's. your head needs to have and output jack that gives the same resistance (ohms) as the cab you're using, or you might blow a speaker. also, volume is mostly a function of speaker size (which directly influences how much air you push), not output in watts or resistance in ohms. i think you should just buy a 1x15 that matches the ohm rating of the head you want and can handle about the same or a little more watts

Well I'm not an expert on ohms, but I do think that you take the average ohms of the cabs (in your case it would be 6 ohms. So that means that you would only be able to run at 4 ohms.

6 ohms is used by many companies (primarily Pioneer) as a "trade off", so the speakers can be used with either 4 or 8 ohm heads. One thing with speaker impedance that you have to remember is that while 4 ohm speakers will sound louder, they also draw twice the current from the head. If you use 4 ohm cabs with an 8 ohm head, you risk blowing bottles or output transistors, as the case may be. Tubes heads will be more tolerant of too low of an impedance than transistors (converse is true for too high impedance). For bass guitar, you also kill your bottom end with tubes and dynamic range with transistors with too low an impedance. Match the cab to the head, it's your best bet.

It's impossible to say which is louder, with only this much information. First, you have to consider how much power your amp will be putting out into each cabinet. Solid state amps will put out more maximum power into a lower impedence, so a 4 ohm cabinet could get a maximum of 1.5 to 2 times as much power as an 8-ohm cab. (Remember, though, that's the _maximum_ power the amp will put out--most of the time you're not using anywhere near the maximum!) If you have a tube amp, though, you get the same power output no matter what the speaker impedence. Second, you have to consider the efficiency of the different cabinets, and this is a really significant factor. There are 2x10 cabinets with effiency as low as 93 dB/w/m or so, and as high as 103 dB/w/m. That represents a big difference in volume for the same amount of power--or conversely, it takes a LOT more power to get the same volume out of the less efficient cab than the more efficient cab. A difference of 3dB represents twice as much power, and a difference of 10dB is 10 times as much power! If the 8-ohm cab is 3dB more efficient than the 4-ohm cab, it will take twice as much power for the 4-ohm cab to put out the same volume (all other things being equal, of course!). There's no direct correlation between power handling and efficincy, so the fact that one cab handles 200 watts and another handles 400 watts doesn't really tell you how loud they are. Mike

As a side note, you find many TBers who use two cabs here. They usually max out the speaker load at their amp's highest rating in order to take advantage of the full potential of their rig. In other words, if they have an amp that puts out 500 watts rms into 4 ohms and 300 watts into 8 ohms, they go out and buy two 8 ohm cabs rated at an appropriate wattage. When they run both cabs at loud gigs, they get the full benefit of the amp's 500 watts, or they can also run the amp at 300 watts with one cab for the softer/smaller gigs. A search of this forum's previous posts on cabs will yield a diverse mix of preferences... two 2-10s (my preference), 2-10 and 1-15, 2-10 and 4-10, two 4-10s (yikes!), 2-12 and 1-15, two 2-12s, 2-10 and 1-18, etc.

Which I was assuming. For "rule of the thumb" purposes, a full double in wattage, or a split in ohmage should offer you a 3 db increase, if the cabs are identical/equal in efficiency.

Efficiency, you are rating this in dBs. Looking at an Ampeg SVT-15E, it has 2 ratings in dB. Maximum SPL 123dB and Sensitivity 98dB. (Also 8ohms and 200W). Are either of these the efficiency rating that you are talking about?