How to balance tube on top, solid on the bottom?

You need a volume (level) control in the more-sensitive amp. If it doesn't have one and you can't add one (and it CAN be done), you'll need some kind of active crossover. If you do that, you'll have to figure out how to drive the speaker AFTER its passive networks.

You also might consider the passive RCA-jack-type attenuators that are used usually to effectively reduce a preamp's gain for better matching with hi-gain poweramps and/or hi-sensitivity speakers.
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Shogun, a few more comments.
1. I believe passive biamping, that is, using the speakers' passive crossovers, is worth the effort.
2. Many speaker systems include networks to equalize response and/or impedance; active filtering which avoids the crossover networks also avoids the benefits of these equalization networks.
3. I believe biamping works best with 3-way speakers with just bass (and not midrange too) on the bottom. The woofer can be driven very succesfully by a solidstate amp. You then can benefit from the glories of a tubed amp on the MR and treble (and not just treble as in a 2-way).

Which speaker? Do you have a schematic diagram of the crossover?
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