Preamp Suggestions??

Hi. I've recently begun moving from the "big box" crapiophile-level equipment to mid-fi audiophile equipment. I am a 2-channel listener only, so no A/V needs. I currently use an Adcom GFA-555 200 wpc power amp, an Arcam Alpha 8SE CD player, an Adcom GTP-500II preamp/tuner, and KEF Reference 103/4 tower loudspeakers. I use fairly nice Audioquest solid speaker cables and interconnects (Glowpower Zyxt interconnects from preamp to amp). It has been suggested that my next step up the hobbyist ladder should be to upgrade the preamp & move to a separate tuner. Again, this is just hobbyist fiddling, as the system sounds great as is. Is there a particular standout in the budget (under $500 used) preamp market?? Many thanks.