Sub-Woofer?

Understood. A subwoofer used appropriately won't shake the floor much. Instead, it will provide seamless extension from your mains. RELs do this very well.
Happy hunting!
Regards
Al 

First upgrade the SA-12.  Buy Nichicon I think 30,000uf PS caps (I think the blue stock ones are 20,000uf so go the extra size just be sure they fit in there.  The upgrade the diodes to HexFred, add a filter choke, and then add Caddock output resistors, check the values on the ones on the AC connectors.  Then make a decision on adding a sub.  Happy Listening.  

If you're patient you can score a great REL used. I bought a Q150e for 200 bucks (in great shape about 5 years ago maybe) then a little later found a minty Q108II also for 200 bucks. I doubt newer RELs sound any better than these things, but I have no idea. These prices might be a little bit of luck, but they show up often enough I've actually badgered friends into buying used RELs when I see a good deal. I make the the Neutrik "high level" cables myself as it's easy, and I use Canare Star Quad wire because it looks good and works great. The bass in my listening room is astonishingly musical, and easily adjustable.

You can't go wrong with a REL. I have an S5 SHO and it blends beautifully with my PSB T3 towers. Properly dialed in,  the REL will add that final sense of space and delicacy in the lowest octave. Good luck! 👍

Thanks, guys! 

Bigkidz, I actually had all the capacitors in my preamp, amp and speaker cross-overs replaced. I don't know what brand he used, but he tells me they all had upgraded capacitance.

Chazzy and Wolf, I am keeping my eyes open for  used REL on ebay. I had actually bought a T3, but it showed up badly damaged, so I returned it. There a 201E and a 150E for sale, at
https://www.ebay.com/itm/REL-Acoustics-Q201E-Subwoofer-Sub-Bass-System-Home-Theater-Made-in-UK/39199...
and
https://www.ebay.com/itm/REL-Acoustics-Subwoofer-Q150E-Sub-Woofer-Speaker/253514671915?epid=17245186...

Neither seems to be in great condition, but they say they work well. What do you guys think?

Gasbose