Hi Mike67, I'd like to second a vote for outlaw audio as an amp. You can get their 770 amp which will give you 7 channels at 200w each at 8 ohms, or 300w at 4 ohms. It will set you back only $1800 of your $8000 budget.
Outlaw Audio 770 amp Outlaw's amps are identical to other, more well known, companies but their prices are usually much lower. And I do mean identical, not just similar. All that is different is the case. This amp weighs 90lbs and means business. I was going to suggest that you look at an emotiva amp from AV123 but they are back ordered. That amp weighs 115lbs and is rated at 400watts at 2 ohms. You do need an amp that can safely drop down to at least 4 ohms. I've heard that Martins often drop down to 2 ohms, that's probably waht is shutting down your pioneer.
THX certification means that the product in question meets the minimums established by George Lucas's Skywalker Ranch sound studio. It is more of a measuring stick than anything else. It can be pretty pricey to put that badge on a product. Because of this some companies will avoid putting the badge on some products that could easily be certified in order to keep the costs down. The Denon 3805 for example could most likely easily be THX certified, but putting that badge on the side would push the price up and make it a less competitve product. THX ULTRAII is more demanding than just plain THX certification. You will usually only find ULTRA badges on high end products that are designed to fill a large space with sound. Actual specs can be found at THX.com.
It depends on what you are doing with your sysem on just how important matching you centers and fronts is. For movies and home theater it is not 100% essential, but I'd still suggest it if you could. It looks better, and if you can drop 8 grand on an amp, and have got Martin Logans out front, I think you owe it to yourself to pick up a nice Martin Center to compliment the others. And you owe it to us to take a picture of your system and send it in.
The AB & D amp question is out of my depth though, I'm sure that the D amp is a digital amp, and that while there used to be a real problem with distortion in these amps they've come a long way recently. I also know that they're small compared to the traditional amps you see most often in receivers and seperates, but that's all I know.
Good luck with the purchase.
