Newbie - Which USED Amp & Preamp for under $2500

At your price point, I would seriously consider(used)...
Preamps:

Modwright swl 9.0 se ($1k-$1.2k) - 6Moons review
EnjoyTheMusic review
Positive Feedback review

Do a search here and on Audio Asylum, this preamp has gotten a lot of good reviews by users.

Art Audio VPS-DM - fully dual mono signal and power supply design. This is a fantastic preamp (1.5k)

SS amps:

McCormack DNA-125 ($1.2k obo)
EnjoyTheMusic review,
Stereo Times review,
Stereophile DNA-225 review(same circuit design, just more power)

Belles 150A Hot Rod or Reference - ($600-$1.6k)
Soundstage review (Reference)

Might also consider Bryston 4B-St or Odyssey Stratos monos

If you are open to newer Class D amps - Wyred 4 Sound SX-1000 or

Channel Island Audio D-200 ($1.3k)
Positive Feedback review
Dagogo review

Mmrump, I actually owned the B&W 601 s2 about 5-7 years ago. They were a great value buy. A little shy in the lower mids & bass(as nearly all affordable small monitors are), a tad hard in the high freqs for my taste (I prefer soft dome tweeters), but a surprisingly balanced speaker for that price range. Your 602's have the same configuration & tweeter, except a slightly larger mid-woofer, so the mid-bass/bass is probably a little better.

The collective suggestion to upgrade speakers first makes sense, but I think your B&W's are better than your amp & preamp. By that, I mean you will be better able to accurately evaluate new components through them than you will speakers through that amp & pre. Although, I think the Haflers, run at the low end of their range, may do less sonic damage than the Adcom.

At the price range you are looking at, I'd venture that it's fairly safe to say an Odyssey, McCormack, Belles, or Bryston will perform fairly well with whatever you eventually get speaker-wise. There will be differences in their overall sound, but all are extremely well made and respected designs that withstood the test of time. They are safe bets that hold their resale value, if you find one of them not to your liking. Someone else mentioned Monarchy, their SE-100 deluxe monoblocks are also an amazing value. Well worth considering unless you need a few hundred watts per channel.

Although, I think the performance/sound in preamps varies a lot more than SS amps at that price range.

BTW, are you going to stay with monitor speakers when you upgrade? How big is your room? What type of music you listen to? And how loud?

All those things will help fine tune suggestions for you.

I don't have space to place them out 2-3 feet from a wall

Maybe some cornerhorns like classic Klipsch? They are big and rock hard. But get tucked into corners so they are out of the way. The only problem is that you would probably have to find some locally because they won' be an easy ship.

Here's some Speakerlab Klipschorn remakes for $800 in central, IL.

Those Paradigms are a great suggestion, too.