In my opinion EQ is much more likely to help than is bi-amping. In your situation you need to be able to make finer adjustments than simply changing the relative levels of woofer and tweeter. That would help, but I think you’d still have weak upper bass/lower mids relative to the top-end of the woofer’s response.
Regarding SET or OTL amps, I looked at the frequency response and impedance curves in Stereophile’s review of the VSM. Relative to a solid state amp, a high-output-impedance (low-damping-factor) tube amp will put out more power into an impedance peak, and less power into an impedance dip.
So in this case the result would be more output south of 100 Hz; a bit less output between 100 Hz and 800 Hz; and a bit more output north of 800 Hz. Imo ONLY the increased output below 100 Hz is likely to be beneficial. North of 100 Hz, I think the net effect on frequency response would be counter-productive.
While I am a big fan of these kinds of specialty amps (I’m an Atma-Sphere dealer and design my speakers specifically to work well with them), I don’t think this is the solution in your situation. I’ve heard Merlins driven by a low-damping-factor tube amp and to my ears the combination still had the characteristics you have described.
Regarding adding a bucking magnet, I assume that "raises the output by several dB" comes from increasing the motor strength. I do not think that is the solution either, for reasons described in my previous post.
I think EQ is probably your best bet for getting the tonal balance you want from those speakers.
Duke