How good can it get, really? - my stereo sounds amazing!

tomic601, you said, "One problem with subs is - there ain’t much music down there... listen to just them....."

That seems like a strange comment coming from someone who uses Vandy 7's, which have low end response of 22Hz  +/-2dB, or just under 20Hz  +/- 3 dB. The speaker has built in subs. Vandy even calls them by the fancy term "Fusion Subwoofers". In fact, it's quite a complex subwoofer. 

An awful lot of subs reach down from about low 20's to mid teens. Pairing the OP's bookshelf speakers is an immanently sensible thing to do, although I recommended he would be far ahead by getting rid of the bookshelf speakers and getting some serious floor standers. 

Re resonators, Helmholtz resonators have to be of the correct size and dimension and in the correct location, otherwise they hurt the sound. Same with crystals, tiny bowl resonators, Mpingo disc, anything that resonates must be in the right location and be the right frequency to improve the sound. Empty beer bottles on the floor HURT the sound, not improve it. In the case of CD jewel boxes they are not the right frequency and there Hundreds of them. Hel-loo! Do the experiment, take them all out of the room. Report your findings.

Well, just don’t go to AXPONA, lol!!!  In answer to your original question, it can get really, really good.  But it will cost you!

Explain to me how anything that resonates can improve the sound unless you are cancelling unwanted resonances. So certainly you are not claiming these resonators are a one size fits all solution. 

This thread seems to be segueing toward a discussion of room treatments. Okay ... fine with me.   

Furniture affects the sound. If you move your furniture, that affects the sound again. How is moving it not a method of room treatment?

Step one: Place your speakers so you get the best sound out of them. It can take a while to get it right.

Step two: optimize whatever else you can, given the room you have. Clearing it of CDs does help, so does moving bookcases and carpets around (it doesn’t have to be far), so does paying attention to the floor in front of your components and between your speakers. An empty floor is bad — Try putting something there. Really ... Try a few different things. (I settled on a teak chest.) And then re-think you speaker placement, because doing step two carefully should send you back to step one. 
Step three: isolation and vibration control. Try different products and approaches. I’ve got isoacoustics footers under my Ref 3a Taksims. I like them fine. Much improved bass and air. That has nothing to do with room treatment, but best to get this factor out of the way before taking up ...
Step four: room treatments.  Look for ones that help. Good luck with that. The best I’ve found have offered only incremental improvements and haven’t been worth it, relative to everything else I’ve done. IMHO looking for room treatments is chasing moonbeams. If your experience is different, happy for you. I’ve heard incremental changes at best, and changes aren’t usually improvements. Diffferrent is not often the same as improved.

finally,

Steep five: play Muddy Waters, Folk Singer all day. Because sooner or later your system’s real stumbling block in going to turn out to be the quality of the recordings you play through it. And that’s the best recorded album I know. (Second place: Laure Favre Kahn playing Reynaldo Hahn.)