I have a fair amount of first-hand experience with the Adcom GTP 760, and it's a pretty good pre/pro for the price (FWIW, Anthony Cordesman gave it a good review last year in "The Perfect Vision" mag). The 760 does not have either 6.1 or 7.1 surround capability, but it does a competent job with 5.1. Adcom's newer model, the 830, which is a somewhat down-scale version of the 760, does support 6.1 if you want the additional channels.
As others have posited, Adcom gear does not seem to hold its value very well, possibly for the reasons suggested above. It may also be due to the fact that several years ago Adcom had some real QC problems with the GTP 740, the predecessor to the 760. Adcom had changed its manufacturing from a very good assembler on Taiwan to a factory in mainland China, and got hit with some QC problems during the transition. Those problems seem to have been rectified, however.