I'll start this off with some basics and maybe post more as it occurs to me. Over the last year or more I've been experimenting with a lot of options regarding sound quality with various ways of running my HT. I started using a Pioneer Elite VSX-56TXi receiver that I bought used here a couple years ago. This receiver was about $1,800 when it was new, 6 or 7 years ago, so it is far from a cheap piece of equipment. I have Thiel speakers in the front and use a very powerful Aragon amp for L&R that provides roughly 500 WPC. At first I was using the Pioneer to power the other speakers, which was a 5.1 system at the time. Actually, at first I had a Mission speaker for a center, because Thiel speakers are stinking expensive and the Mission made a decent, though hardly perfect match. The first interesting thing I found was that when I bought an Emotiva XPA-5 amp for everything other than the L&R, the Mission center now matched the Thiels far better than before. Not only does the Emotiva amp have more power, but it is a different basic design. Receivers ALL have class D amps, and the Emotiva is class A/B. I tend to be skeptical of the capabilities of EQ stuff like Audyssey. I see no way they can create the same difference as you get with a better amp. Wikipedia has a decent page explaining the basic differences in amp classes. The basic rule of amp classes is, high efficient class D amps are also complex and have relatively high distortion, but their big benefit is their efficiency. Then the designs step down in efficiency and distortion with class A/B and then class A. What I don't know is if the improvement in matching between the Thiels and Mission was due to the added power, better amp design or, most likely, a combination of both. The Aragon amp that drives the L&R Thiels is even a step above the Emotive, running Class A up to at least 100 watts (with the Thiels' impedance of abour 3 ohm) and A/B beyond that. The downside is that class A amps are absurdly inefficient at about 20-25% efficiency. Class D amps are usually at least 90% efficient. The problem is, I think (depending on your speakers) there is a genuine sonic difference. I suspect this is a big reason people hear such a difference when they go to external amps. It's not just the greater power, but a "smoother" sound from A/B amps vs. D. The next thing I tried was a newer receiver as a "Pre-amp" for the system. At the time, all the Marantz models had full pre outs, so I could use one in place of the Pioneer. So, I got a SR5005, the bottom of the line, for $500 (it's only $450 now) from accessories4less.com. Remember, I am using ALL external amps, so the receiver is really operating as a pre-amp. The surround sound was noticeably better than with the (older, but significantly more expensive) Pioneer. By "better" I mean on par with the difference between DVD and HD sound with BR. It wasn't subtle. What I don't know is if it is because the Marantz receivers are that good, or just that surround processors have improved that much. Once again, I suspect it is some of both. So, one of my conclusions is that it does make sense to use external amps, but use a receiver as a pre-amp. It seems silly, but pre/pros are expensive and if you can get 90% of the improvement with a good receiver that has pre-outs for the external amps it is more practical in the long run, especially since you can move that receiver to a smaller system when you upgrade. Today, I broke down and ordered Emotiva's UMC-1 pre/pro, which is on closeout because they are introducing a 3D compatible version. I guess I'll see if I hear an improvement over the Marantz receiver. Still, I was surprised what an improvement the Marantz was over the Pioneer when using external amps.
