Subwoofer Suggestions

You'll probably have to get the sub signal from the speaker leads, so just about anything will work. 

Plug the ports and try to keep the sub crossover high, 60-80Hz.

I find with subs that an EQ is almost 100% needed to clip peaks, so if you can find a sub with built in DSP that's a big plus.

Check out the AM Acoustics room mode simulator to help you place it and the speakers.

I think  you'll get better results with two subs, not just one.

You might be able to do that if you buy used.

Two subs, each one beside each main speaker, worked wonders for my system.

I suggest Rythmik. As good as REL and less expensive. Try here.

I tried 5 different subs (all under $400) and could only afford one vs 2. I lucked out, the one that worked out the best is a 10" Polk from shopgoodwill for $25. Everything else was a giant mess, sound-wise. I am looking to score another Polk. I am using the speaker connection, the experts told me that's the best (besides that my amp has no sub out).  

Unless you have an expensive setup, I wouldn't "outclass" it and outspend on sub(s) with REL or SVS.

I also tried managing the frequencies with splitting it between the sub and speakers but of course that costs money and I wasn't that lucky with my purchase in that area. 

You could also search the forum for previous discussions, there has been a wealth of knowledge and case specific advice here, all I know I learned from here. 

I have owned many subs in my lifetime, and at your price point, I recommend the RSL 10S MK II. I own the 12S, and it replaced a pair of high quality subs (I will not mention the name here, as I do not want to offend anyone or any brand). My best, MrD.

No love for SVS here? I'm also looking at the same choices and it looked promising. Is there an issue with them? 

Everyone has their favorite brand. SVS are fine subs.The equalizer has an app that you can install on your phone or tablet and do all the adjustments from your listening seat.I'm very happy with mine.

For 600 used is your friend. Bought a little 10 inch velodyne that’s 10 years old for 300 bucks. Great little sub that was around 1500 new and punches way above its weight. 

Agree with others if you can get 2 for your budget even if mismatched will be way better.

nothing wrong with SVS. I don't think the entry level models are the best value or at least they did not work for me.

Would the B&W 608 be a good option? Fits in my price range.

It’s a powered sub…don’t know if that causes any compatibility issues with a vintage receiver.  I’m way out in left field with this particular thing.

I just looked up that Receiever. You need a sub that will accept speaker cable from the amp or make one with rcas and I think a resistor. Powered subs didn’t exist back then. Rel does that not sure who else. 

good point @mofojo most 2000s', 2010s' subs have a direct speaker input 

Make sure the sub you choose hs line level inputs. Do a search to see the wiring diagram. This takes the hard to drive low frequencies load from the receiver and uses the powered subs amp to drive the lows. I think this set up provides better dynamics for any system.

Optimus has some decent sealed passive subwoofers that are inexpensive and have speaker level inputs and outputs plus built in crossovers. The model number is SWS-500. I have 2 I will sell you if you’re interested.

Get 2 of Vera-Fi's 10" subs.

REL Tzero or T5x?

REL.   Best speaker wire interface.   T Series are great little subs.