New TT or new Cart? Technics content

Absolutely without a doubt a second subwoofer! Next would be a new cartridge but I would shoot higher. A Grado Ethos or an AT ART9. 

You have to completely rewire your Technics first (internally too), then add Isonoe feet and SAEC SS-300 mat, and try mid or high compliance vintage MM (not MC) cartridges - this is the best for Technics lightweight tonearm. Upper models from Grace, Stanton, Pickering are absolutely amazing on this tonearm. I have the same turntable, look in my system. If you need advice for a killer MM cartridge for this tonearm send me a message, I will be happy to help. I have AT cartridges too, but better models. Tried over 50 different cartridges on this tonearm/turntable, still use this TT when I test cartridges (I have better turntables in my main system). I tried a few $3-5k LOMC on this turntable and ended up with vintage MM. Technics MM cartridges were very nice on this tonearm. 

Actually your fluid damper designed for High Compliance cartridges, your AT is mid compliance.

Try to find superb cartridge first, you can change turntable later.

The weak points of old technics are poor wires and feet.

Both ideas above are very good. But I believe a separate phono preamp should also be a consideration. The question then becomes which suggestions to do first.Id be inclined to do @chakster first if yo plan on keeping this TT. Then it's a toss up. 

The TT has already been upgraded:
Feet, dampening, tonearm rewire, outboard power supply.  I've done it all. 

Really no interested in going back to separates.

So, cart or extra sub it seems?