With respect to Rgd's point on the problem with multi-channel discs, I can report no problems so far with my modded 777ES, which is a two-channel player, knock on wood. By the way, my friend is now looking seriously into mods--I had not been able to hear my modded 777ES vs. a stock one head to head till last night, and it was a revelation both on SACD and redbook--probably some of the best money I ever spent in this hobby. What a difference a better digital clock, a high-quality output stage (tubed, in my case, but not the point) and a separate, no-holds-barred analog power supply can make!
On which model to get, I tend to agree with Nrchy's point, principally because the transports on the more expensive units seem to be much more robust and well-made and have better power supplies on the digital side. However, part of your decision will rest on which modifier you want to use and which models he works on. I know that Stan Warren had a mod for the 9000ES which reportedly was very good, and very cost-effective as well (in the $250 range, I think). Good luck--I think you'll be surprised how good a custom-made piece (that's what these mods essentially are) can sound against any stock unit.