You're right, Doug....
I don't own an equipment rack or isolation system yet, so I actually have to place my turntable down below in the dining room and use long IC's to get it away from LF energy right now. When notes hit those room mode frequencies, the panel in my fire place starts to buzz and the windows vibrate.
So, it's not really worth it(using TT) for me until I get a good rack and room treatments. Getting up to run downstairs every 20 minutes or less is beyond silly.
But, having said all that, the difference a sub (or subs) make in the texture of bass and midrange is astounding. My main speakers are similar to yours in FR(~40-36kHz). Though, in reality, I'd bet you've got a lot more 40-60Hz output than mine.
Anyway, everything is much more fleshed out. There's just so much more "body" to instruments and vocals it is surprising. Not in a "slam" or "punch" style, but the actual resonance of the instrument. Tonality and texture have jumped into the next league.
I can't speak to spatial information yet because of my room problems, so others will have to jump in for that. But, suffice it to say, I cannot stand to listen to my mains anymore without the subs. They sound weak and anemic in comparison.
Pretty surprising considering two weeks ago I thought they did a terrifc job of producing deep, tight, bass for horns. I still do, though nothing close to what I've got now.
But, since you've already said in your system description that the Salamander Synergy Triple 20 is "Too resonant for a serious audiophile, but looks nice in the LR", it's a safe bet that you'd have to get a new equipment stand or a lot of isolation platforms, blocks, rollers, pads, etc. Then, there's the inevitable room issues to keep you occupied for a good long while, especially with only on spot for sub placement.
Aaaw, screw it - go for broke! That's what this silly hobby is all about - spending money and chasing that last 5% of performance. You'll never know what your system is truly capable of until then(just joking).