I have the 501's and have been using them with an older ARC LS2. I have since purchased a Mark Levinson 390s with a direction balanced connection into the amp with incredible results (makes me want to bypass using a preamp altogether). However, I do plan to upgrade my preamp as I am incorporating HT with my 2-channel (swapping IC's gets old after a while). So far I've have been considering the BAT VK-51SE and Aesthetix Calypso. The 501's are wonderful sounding amps, and having heard them without a preamp (depending upon what your goals are) I would look to find the most neutral and transparent preamp that you can afford.