Used ARC Ref 75 versus new Pass XA 25 versus new Parasound A21+

Here's my situation:

Have cobbled together some left over gear for a second system at my mountain cabin.  At minimal cost (bought most of the stuff many years ago and since upgraded), the gear is as follows:

Totem Rainmakers on 24" stands
Late 70's Pioneer PL600 turntable (the good one) with a Hana EL cartridge
Naim CD 5 player
Classe Audio DR5 II preamp
Adcom 555 amp (original version)
Cogelco/Monster assorted cables

My thoughts are that the weak link is the amp, by a considerable margin.  Need to find a versatile amp that will improve the sound without breaking the bank.  The above choices are available to me at $3K to $4K and while all would work any thoughts on which would be best?

I thought about even less expensive options used but then again want to make sure an upgrade is achieved - otherwise I'd just keep using the 555.

Thanks for the input.