I can only comment on the TF750a and the Max 9.2. From what I understand, the primary upgrade with the TF800 concerns cooler 2ohm operation, so I assume that the 750 and 800 sound pretty similar at 4ohms.
Right out of the box, the Max is much brighter up top, and a bit thinner down low. However, I could get the Max 9.2 to sound VERY similar to the inherent 750 tone with EQ (primarily bumping low mids and cutting upper mids), but could not replicate the brightness and articulation of th Max 9.2 with the TF750, which always sounds pretty relaxed and warm. Both are great heads. At 4ohms, the Max 9.2 definitely has more wump, and with the variable hi pass filter dialing in more deep low end, it will outperform the 750 in open, non-compressed low end down low at high volumes.
Both are great and have similar EQ power. The TF will work at 20hms while the Max 9.2 has more wump and heft at 4ohms but will not do 2ohms (that is what the Max 12 is for).
Both top notch. I found the Max 9.2 more capable of a wider variety of 'voices' with the powerful variable filters, the FET and Tube preamps, etc. I'd be pretty happy with either.
IMO and IME.
Edit: Given that the '800' puts out 630 watts at 4ohms, and the Max 9.2 achieves its max power rating through some pretty slick circuitry/power management, neither performs at their 'nameplate wattage of 750watts and 900 watts respectively IMO and IME. However, both wump pretty hard, with the Max 9.2 having the edge at 4ohms. Interestingly, as an example, my Glock Blue Soul uses a 1000 watt class D module (can't remember the make... not the typical ICE module) and is rated at 650 watts, and it buries almost every other amp I've tried in open, wide, clean, pure, non-compressed low end (of course, you might or might not like the tone, like with any amp, just talking about pure performance below 100hz). So, more going on than the pure power spec's of all these heads.
Thunder funk has a nice thread about power and ratings that you should read. Many things here can be debated. Dave stated below.
The prototype TFB600-B2 is owned by the Deputy Attorney General for the State of California. He and I are the only ones who have heard it. Anyone else's comments are just guesses.
Amps have to deliver a voltage into an impedance to generate power. Ultimately the max power is the max available voltage. When you try to short out the amp (2-ohms) the voltage collapses as the voltage drop through the windings increases. If you can double the power by reducing the impedance, why not run at 1-ohm? Or 0-ohms!!! Wow, a ton of power right before it blows up!
The comment about the 550 delivering "significantly less power than a 550", uh, 530 is significantly less than 550? I've said this before (many times) twice as loud as 550-watts is 5,500-watts. Significantly less?? It's the difference between 46-volts (530-watts), and 46.9-volts (550-watts). Barely a difference, but guys get all concerned "It might not be enough power!" That’s why I make the 800. Speed is just a question of money. How fast can you go?
But you hit on something that most guys don’t know about. How many watts at what distortion level? I measure mine at the level that the tip of the wave starts to change. If I measured it at the level that it started to show flatness you could easily add 50 to 100-watts to the rating which wouldn't even be noticeable as to "loudness."
The 550 has 4-output transistors. The 600 has 6-output transistors. This lowers the output impedance, spreads the current load and heat load. A lower output impedance means the amp has a better damping factor which means the amp controls the cone movement better, which means lower distortion, ringing, etc. I never thought that adding two more transistors would affect the tone so much, but it does.
As to ratings it actually puts out 650-watts at 2-ohms. And it can actually hold that power level continuously. Digital amps cannot. This is a rarely discussed fact that almost ALL digital amps don't have any significant amount of power filter caps. No reserve energy. They are designed for 600-watts of Peak Power with the assumption that the demand for 600-watts will be fleeting. That;s why they don’t thump. I've heard one engineer say that if you try to test a 600-watt digital amp at a continuous 600-watts, it'll last for less than 1 second before it blows, and another engineer said it’s 300-milliseconds. The comment was also made that in order for a certain brand of amp to deliver its rated power it'd have to be 125% efficient. Obviously someone’s lying.
And there's another factor that I mention in my demo video. Phase coherence. When the preamp is delivering a signal to the power amp that's all jumbled up due to the use of electrolytics in the signal chain, the amp self-cancels itself. You get 600-watts of a signal that can't penetrate the room.
B&O says the average power is 13% of the peak. I estimate it’s more like 10%. So with an 800-watt amp you’re actually only using 80-watts. The rest is dynamics. That’s why a tube guitar amp can be loud at 100-watts. If it lacks power and distorts and compresses (sings/sustains) guitar players love it. Bass players hate it.
And finally, the last thing rarely thought of is amps are typically tested at 1,000 Hz, but what's the power rating at 50 Hz? I don't mean power fall off due to EQ. I mean the ability of the power amp to deliver power at a low frequency.
So, how much power?
1. At what distortion level?
2. For how long?
3. In phase?
4. At what frequency.
Without mentioning brands () I have two more comments.
A famous digital amp was brought in for service in Australia, and the tech said he already had five of them in storage that couldn't be fixed because the part numbers were all removed and the builder won't give out schematics.
And when production of a world famous brand was moved to China the warranty was changed from 3-years to 1-year. It may be the same schematic but it's not the same quality of parts.
And finally, I have to laugh at how threatened large international corporations feel by one guy building amps at home in America. I don't do anything they couldn't do. I've worked for more than 30 companies in my career as a designer, and I'll tell you what happens. An engineer designs a power supply, someone else designs a preamp, and another engineer designs a power amp. They put them all together and the bean counters take the money out of it. Whereas I do whatever I want to do. The only one I compete with is myself.
