It's been a few years since I originally posted in this thread, and I'd like to share what I ended up doing, along w/ some related observations, because I have solved this problem to my complete satisfaction, and perhaps someone else will find this information helpful.
Once you have the capability to fine tune the level of the bass amp relative to the mid/treble amp, you really come to appreciate the control.
Completely agree. This has been a guiding principle for me. I did not want any unnecessary component, not even passive attenuators, in the mid/treble signal path. Any adjustment should, ideally, take place on the solid state / bass end... but I was unable to pull this off, until, about two years ago, I found an add-on device to balance things out (see below).
Gear: I'm now on my second pair of VSAs, in this case VR4 SR MkIIs which I am in love with. Also, my tube and solid state amps have changed since I last posted... I now have a Cary tube amp on the top end and a Wyred4Sound solid state amp on the bottom.
A note re: the tube amp -- before I obtained the Cary, I had a Dared amp, and to achieve the correct balance between it and my solid state amp, I used passive RCA attenuators... I tried two different brands and a variety of db levels... since they were more expensive I expected Rothwells to sound cleaner, but to my surprise, the less expensive ones from Harrison Labs were a better fit for my system -- they simply were invisible, which is to say, if they added any sonic "veiling" (Albert's term) I could not hear it. However, once I switched to the Cary tube amp, I no longer needed any passive attenuators at all.
A note re: the solid state amp -- the Emotiva was a bit mushy, so I ended up going w/ a Wyred4Sound amp instead. I also have a really nice Parasound power amp which I've been quite happy w/ at my office running full range (i.e., not part of a bi-amp setup). I briefly tried it at home in place of the Wyred4Sound, and didn't care for it at all. In fact, I tried it on two separate occasions, because "on paper" it seemed to me that the Parasound was the better unit. Point being, do not assume all solid state amps are created equal (despite having similar or identical power ratings).
One other piece of gear worth mentioning is my pre-amp, an AR SP-16, which has a dual pair of outputs, i.e., perfect for bi-amping VS-es. And I'm guessing that for anyone who cares enough to bi-amp their VS-es, I don't need to point out that it's worth experimenting to find the right interconnects, especially the mid/treble pair.
Okay, having shared every bit of background info I can think of, here's what I ended up doing. As stated at the outset, my goal was to do all the "heavy lifting" on the bottom end, to allow the mid/treble signal path to be as simple as possible. So, I ended up purchasing an Eastern Electric BBA ("booster buffer amp")... the company is no longer in business, but you can find used BBAs typically in the $400-$600 range. For more information, see Doug Schroder's review here (https://www.dagogo.com/eastern-electric-minimax-bba-preamplifier-review/
), though of course he was not contemplating using it as a bass adjustment component in a bi-amp scenario.
What I like about the BBA... 1. it's an amplifier, so it can boost the signal, as opposed to merely being a passive attenuator, and 2. it has both a "gain" and a "volume" control... I don't know what's going on technically, but in my system, "gain" equates to "punchiness" and "volume" equates to "boominess", and I tend to keep both controls in the 12-1 o'clock position, though depending on the recording I will sometimes go up to 2 or 3 o'clock w/ one or the other control. If I found that I was routinely wanting to push either gain or volume into the 4 o'clock range or higher (5 o'clock is the max), that would be an indication to me that it was time to insert a pair of passive RCA attenuators into the signal path between my pre-amp and the tube amp. (Which I had to do w/ the Dared tube amp, but not w/ the Cary.)
Between the gain and volume controls I can "sculpt" the bass to my liking on a track by track basis. Typically the adjustments are subtle (ranging between 12 and 1 o'clock), but the result is NOT subtle... I find that oftentimes the difference between "okay" and "incredible" is a small adjustment via the BBA.