midrange forwardness

Chris,

ATC will give you close to the best "electrostatic sound" with a tilt towards the lower midrange that is missing on some panels. Not as perfect in the midrange as you can get from the best panels but overall much better dynamically (not laid back) and an excellent all round choice because of the bass and treble. Gordon Holt used large Soundlabs for many years and more recently got active SCM 50's. Both are excellent speakers but ATC is more dynamic and forward, and seems to have more emphasis in the lower mids (male vocals come out very realistic).

So why would the Soundlab sound relatively louder in the treble with a solid state amplifier?

It won't.

It will give you more bass (where impedance rises) - so if anemic bass is your problem then a tube will help.

If you want more midrange then a tube amp will unfortunately reduce the output but you may get some more harmonics in the mids (more warmth) - if that is what you need then tubes are a good solution => more bass relative to mids and more warmth overall. (note warmth is related to loudness in a sense so although mids may lose some power they may sound more "rich")

Good luck!