Killer used integrated for

   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.

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

You're killing me.

No way 

Well, you can likely find a tube integrated for that (I know--China--you take your chances, OR a reconditioned receiver--again you take your chances), which (if it works) will likely (again!) sound great, and even if it doesn't (some folks are anti-tube), will be interesting and amusing (isn't that the point of a hobby?), and likely serve as a nice discussion point for anyone who sees or listens to it.

It would be helpful to know what speakers you will be using in order to make a good recommendation. In my bedroom system I use a Rega Io and Tannoy Mercury speakers. It's a great sounding amplifier that punches way above it's $599 MSRP. However, it requires careful speaker matching as it only puts out 30wpc. Good Luck!

When you say 'killer', if you mean decent 2 channel integrated, then consider the Cambridge Audio Azur 851A. I've seen them used for $500.  Good luck

+1 for Exposure if you can find one. Good stuff.

Not possible ,

Go up to $900 and get a used Peachtree Nova 150. I owned it and sold it after a few years. It served me quite well.

Yup, I would look for British integrated.

Musical Fidelity M2si

A killer integrated for under $500 is a tough ask IMHO.  There’s a NuPrime IDA-8 on US AudioMart for $700 that’d be great if you have an extra 200 bucks to spare.  Best of luck. 

REGA BRIO

Creek Evolution 100A 

Cyrus 6vs 

Exposure 3010S2D