Class A vs. AB vs. D... can you hear a difference?

Linear power supply draws current from the mains in narrow spikes of high amplitude. These spikes contain a lot of harmonics that might induce noise in any LC circuit inside as well as coupling to other cables. I called it a primitive switcher because diodes switch on/off when voltage is the highest. It also produces diode switching noise when diode is suddenly reverse polarized and conducts for a moment in opposite direction to finally snap back. In addition to high frequency noise it also produces 120Hz noise that is very hard to filter out. It requires large transformer that produces mechanical noise especially in presence of any DC. It requires a lot of capacitors to clean 120Hz and to keep voltage steady since it is unregulated. These large capacitors contain inductance compromising amplifier response at higher frequencies. Adding non-inductive capacitor in parallel helps but creates parallel resonant circuit (with inductance of main capacitors) that rings.

Modern zero-voltage/zero-current switching SMPS operate at high frequencies (Rowland latest SMPS operates at 1MHz) that are easy to filter out. In addition they often contain Power Factor Correction presenting resistive load. In addition they are line and load regulated with instant response and are not sensitive to presence of DC (can even operate from DC). Jeff Rowland uses SMPS in preamps where efficiency is not important. Benchmark's new power amp contains ultra quiet SMPS resulting in overall S/N=132dB.

http://benchmarkmedia.com/products/benchmark-ahb2-power-amplifier

Bel Canto wrote white paper highlighting advantage of SMPS

http://www.belcantodesign.com/pdfs/EfficiencyandPerformance.pdf

Jeff Rowland also wrote many papers explaining use of SMPS:

http://jeffrowlandgroup.com/kb/questions.php?questionid=145

I've owned excellent versions of all 3... I've owned a dozen or so amplifiers in my time and can tell a difference between every amp that I've owned, But I can't say that it because one was Class A... A/B or D... Great amps are available in every class output. I suspect that if you put 3 excellent examples of a Class A, Class A/B and a Class D in any system, most of us would be pretty happy campers.
I currently own a very highly modified Sumo Nine and an amp with Abletec Mono Modules that I built in a stereo case.
Both are very nice... By the way, anyone out there that would like to venture into Class A inexpensively, the Sumo dead stock is pretty good, but proper parts upgrade really make this amp come to like, incredible value.
Overall, I believe that you could make an easy enough argument for tube type or bipolar vs Mosfet or any other output device. Overall, its the design that matters.
I hope this helps, Tim

Bill as you can see objectivity can be hard to come by especially when one loses site of the question.

I really enjoy my home theater. It's a 7.1 system in a smallish space with a Pioneer Elite receiver driving sensitive Triangle speakers and two 10" Velodyne DD plus subwoofers.

I've heard some very expensive HT systems that are simply wasted on the quality of much of the media that's available today. In the end it's just TV with a digital source which doesn't even come close to the elegance of analog two channel. Here is were switching amplifiers make sense because they can be constantly powered without wasting tons of volts. I wouldn't stress the THX standard anywhere near as much as getting the extra low frequency right and having the ability to control the ELF on the fly with remote volume and EQ presets.

If you have enough room effective speaker placement rather than depending on drastic room correction usually results in a better presentation.

My system really came into its own when I went to 7.1 and got rid of the dipoles. The transitions from back to front are much more defined even with 5.1 media matrixed to 7.1.

Bill, I'm very sorry that I wasn't objective and lost site of this question.
"Some Class A amps go to class AB after so many watts... is there an audible change?"

A short answer would be, don't worry, a good representation of Class A, A/B or D could do a good job in your home theater.
It is more how the design is implemented than the class of Amplifier. Check the forums and read a few reviews, you'll zone in on a few pieces very quickly. I'm sure that if you would like to advise if your are thinking a receiver or integrated or seperates, you will get some good advice.
Good Listening, Tim

Tim, Please accept my apologies for what appears to be a comment directed at you as that was not my intention.

Without even thinking that my comments were following yours I didn't mean that YOU weren't objective, rather a portion of the thread in general.

Again my apologies.