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

CD jewel boxes, for example, are actually *resonators* and not in a good way

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Kinda curious, how exactly do you define " not in a good way". And like is it something intrinsic to resonators or just that particular resonator.

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.