Not for nothin' but why not a tube preamp instead of another amp entirely?
Personally, that's the way I'd go. Tube preamp.
One note though... there are those recordings which no matter what, simply can't be played on higher res systems. Some just plain suck.
I've found my answer for those sorts is to play them on less resolving systems... My PC for example, my BR HT system.
The bad pressings were aimed at a particular crowd. Geared to be played most often on boom boxes, mono radios, etc. Back in the 50', 60's, and through much of the 70's many Pop, rock, & RB genres were mixed down just that way... for use in car's with one 3x5 or 6 x 9 speakers... and so forth.
Taking them out of their orig element by replaying them on very good equipment, shows up their true nature quite clearly.
Dumb down the rig or use a lesser resolute one to play those bad discs with.... works for me... OR just deal with it.
it's like seeing a pretty girl across the dance hall room. She's great looking from that distanced and dim view. Up close in good llighting, the glamour disappears... sometimes.... or you see better any flaws which are there in fact.
trust me on this, if the source info isn't good to begin with just how is it supposed to get better down stream?
it won't. A tube pre will help some, specific cabling will too, but nothing fixes bad Cd's better than playing them on the stuff they were intended to be played on originally..