Killer used integrated for

+1 on cambridge 851
also, NAD produces some good stuff

   joey54 poses the most important question.....Which speakers will you be using? Many excellent amps are recommended here. I have experienced and observed dismal results with mismatching amps and speakers more than a couple of times. Some amps sound great with some speakers and not so great with others.
   Searching for a "killer amp" in the used market may not be the best path. Since you are looking to upgrade your "basement" system I infer that you may not be looking for a mega watt amp? But I don't know the size and geometry of your basement.
   My course would be to audition possible amp candidates with YOUR speakers. That is more difficult in the used market. Or, read all the input from users on all the forums you can find and try to see if others with YOUR speakers have had good or bad results with different amps. Amp and speaker synergy can't be over stressed. Good luck.

10x the Musical Fidelity m2si.  I loved this amp and should have never sold it.  40wpc is BS.. I don't care what it's rated, it played with the big boys.

Bought an Hegel H100 for 650usd. It’s a bit pricier but nothing compare at such price.
You will get depth, great timings and imaging, soundstage, bass, beautiful midrange and Hegel liquid sound signature at 120w@8ohm and 240w@4ohm. Plus a usable DAC. Good luck if you want more.

Not sure how many watts you need, but Vista Audio's Spark is great.