What's the deal with Adcom GTP 760 going so cheap?

Technology(most of the time) and marketing(all the time) strategy changes rapidly in hot markets, and home theatre is the hottest of the audio markets at the moment, and Adcom and its retailers have done very well financially by staying up with that market, so I would would think that is part of the reasoning your seeing the 760 discounted. Using 1 or all of the channels is fine.

Most Adcom gear is pretty reasonable overall but doesn't seem to hold it's value. This is probably because there is so much of it out there ( rarity or lack of availability raises value ) and people are always aspiring to climb higher up the ladder.

With that in mind, I see nothing wrong with going for the Adcom. Just make sure it has the features that you want up front.

As to an amp, you can use however many channels that you need. In all honesty, using only 3 channels of a 5 channel amp would probably give you BETTER performance and sonics. This would be due to the increased power reserve that would otherwise be "tapped" by the other channels that you are not using. Doing such also lends greater versatility, as you are covered for 7.1 or the use of multiple subs, etc... should you choose to go that route at a later date. Sean ( jamming to Blue Man Group with 6000+ watts on tap )
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I've been wondering this myself, especially as the 760 is still in production and seems to have a pretty good reputation. It may come down to the 760 not supporting some of the very latest in HT standards. But the 5.1 standards are still, well, standard, so the 760 should do fine until you upgrade in a few years to get Dolby Pro Logic v23, etc. and in the meantime save yourself quite a bit of money.