Used audiophile preamp

Perhaps rather than replacing the amps consider an upgrade with new high end capacitors in the audio path.  Many have had success with this approach.  Mundorf Supreme Silver, Gold, Oil (1200 volt versions), Miflex, Dualand, several others.  Find a reputable shop who has worked with your equipment.   Personally this has worked wonders for my vintage tube equipment and some SS as well.  

Take a listen to older conrad johnsons.

Best,
E

If you are thinking of tube amplification, i agree CJ....Quicksilver is great too and huge bang for the buck.   I would also recommend going Rowland amp if you were to make an upgrade and stay SS.    My preamps up until recently proved to be the bottleneck of my system,  My c-j pre sounds fantastic and I'm fairly certain it would be big $$$ to improve it.    I upgraded my amps since and they too were an improvement but I honestly think that pre is so good I could upgrade my other components further and the preamp would not be outclassed.

My point, if you have a great pre there are probably better ways to spend your money.

Conrad Johnson pre's are really what they are know for best, although their amps are incredible. CJ Premier product line is usually Stereophile status such as Premier 16LS; Premier 17LS, etc (not Premier 18 in my opinion) ET5 is newer and in ways superior to their new mid product the ET7 (have not heard the ET-7 series 2 which came out w/ in a year of the ET7 release.  Just some recommendations. The price is now always an indicator as well. The Premier 17LS in ways is less liquid than 16LS, but not in a bad way. Most everyone I know who has had a 16LS, had a 17LS and could never completely decide which to keep so they ended up keeping both because there is the slightest difference. They all sound like CJ pre's though. 

Schiit Freya...an amazing preamp for peanuts.