Killer used integrated for

Setting up a basement system and looking to replace older Marantz surround receiver with something more for 2 channel for under $500. Was looking for a Audio Refinement Complete but none for sale. Any others I should be considering? No phono needed.
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. 
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.
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
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!
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.
   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.
+1 on Rega Brio. I just sold one on another marketplace. 

Also, killer is not the word I would use for any of these.
'Killer' is a matter of perspective!For a good value in new, how about the Yamaha integrated amps in the price range?
I've been looking for something similar. Mine is specifically for a pair of EPOS M5 speakers which are 88db, 4 ohm bookshelf speakers. (Recommendations welcome.)

A few things that have been on my radar are the PS Audio Sprout, the NAD 3020, the Emotiva TA-100, and the Nuprime iDA6 which is on clearance for $350. You can probably get to some things a bit higher than this list if you are looking used, but of course that's dependent on what you actually find available used. 
I found an older Audiolab 8000A for $300 a few years back and it's great. 

For that price used, try it and move it on if you don't fall in love.
Just keep what you have, swap the amps to 2 channel and be good. Not going to find much better for the $ you are talking about. And you get Audyssey to deal with room funkiness.
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You might get close with a Zpre3 Pre and a Zpre V3. 2 channel power amp for close to the 500.00 mark.. If that is an option. Nice package, great options.. Vary compact too. As small as most integrated packages.

OP here. Wow, great suggestions. To clarify a couple issues: using the word "killer" was only meant to identify amps that perform well above their price point. I know performance will not be the same as the big rig upstairs. Second, speakers will be ProAc monitors, not quite sure which model but along the lines of the D2 or Tablette 10. 
Looks like i need to do a lot of research/shopping.

The Outlaw Audio RR2160 mk2 sells for $999 new. I saw a mk1 listed for right at $500 recently.  Its well recieved for its price point. 

I have heard the Rega Io with the Proac D2's and Tablette's. They sound great together. The best I have heard with the Io was the Vandersteen Wood VLR CT's. Not exactly what you would expect pairing a $3,000 speaker with a $599 amp but they sound great together. The standard VLR @$1650 is a good match as well.

If you are anywhere near Verona NJ you can go listen to the Rega Io with the Vandersteen's, Proac's and B&W speakers at the Audio Connection. JohnnyR will take good care of you. Good Luck!
Used better or dual mono Japanese integrated for $100-150, in OK but flaky shape.

Taken to a pro tech who is audiophile oriented and rebuilt to beyond original spec for $350-400.

total, $500, and not going to be equalled for at least a few thousand, if purchased new.

maybe not easily beat -at all.....depends on the amp found and at the core of the rebuild.

but one has to put the work in. No (risking of) pain, no gain.

Also, Sun Tzu: If you sit by the river long enough, the body of your enemy ~will~ float by.

I’m saying that you’ll have to be patient and work for it... wait for it...and chose the right item.
I have an excellent condition Audio Refinement Complete in my closet. IMO a step above Rega Brio R and Rega Mira 3 I have owned. I don’t have a use for it, but keep it because it’s so good.

I currently use Vista Audio spark on my desk. Great little amp but only 20 watts.

Someone mentioned the Parasound Zamp. Also owned. Very clean and analytical sounding. It almost sounded like Class D. I didn’t care for it but I imagine it would be real nice with a tube front end.

Someone also mentioned the little Emotiva 50 watt amp (also owned). It’s good for money but a step below the Rega amps I mentioned.

Good luck!
Rotel RA-980BX. I own one since 1992 and except for some capacitors that needed to be changed 10 years ago, it has performed flawlessly. Clean and simple design, quality built, plenty of power. They do not come up very often in the US but if they do they usually go for less than $500. Here is some info on it:
+1 on the Exposure. I had one here for a while. It definitely plays well above its price. Iirc, they also sell a matching amp in case you’re looking for more power down the road
@mcondo - get a lovely vintage reciever... recently picked up a Luxman R-1050 and its extraordinary! Rosewood cabinet, lovely glowing meters so the vibe is right but, it PLAYS BRILLIANTLY! Can pick something like this up for $5-$600 so right in your wheelhouse. 50x2 of clean bountiful power that's enough for any relatively efficient speaker. Plenty of options but that'd be my route. GL!
I also recommend the Exposure.  Helped a friend find a decent used integrated a couple of weeks ago and found a used Exposure at a local dealer for $500.  Excellent value; no bells or whistles, just quality amplification with analog line inputs.  
Yamaha A-S500. Measures better than some amps costing 5X its retail price and sounds great.
I would look for an old tandberg integrated may pay a little more than 500 dollars but not much.
Another for Rega.Brio is great, or if you want to save a little more and are ok with larger size, the Rega Mira 3 line was really great (what I am currently running).  Not super powerful, but with Bookshelf monitor style, the integrated does great, can run a sub, run as a pre or just amp, remote control, and matches great with their CD player or DAC~r. 
Krell kav 300. It’s slightly more than $500 but definitely has a typical sound signature of Krell. It looks good too. 
I agree with the Rega Brio. If you feel you would be upgrading the Rega soon I second upgrading the amps on your existing system. You can add a nice Rouge RP1 tube preamp for $1500 later to improve on the system and Rouge has home theater pass through so you can integrate it with your surround system. Last  I would be checking out used gear at stores. Most will ship. They take stuff on trade-ins all the time but only list on their own website. The Brio, Rouge, and nice used equipment is all sold at my local store if you don’t have one nearby.
hshifi, I've been to Holm - thanks for the reminder. My son has a Rega Brio 2000 that he is happy with, I'll take look at their lines.