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.
.

Well you will have to have some sort of gain control or sensitivity switch on at least on one amp. Older McIntosh amps had it and probably others do too. Otherwise you need a crossover or attenuators in line but that would be too messy for me.

Many pro power SS amps have sensitivity pots (my Bryston 4B does). You can adjust these to match the level in your tubes.

Biamping is probably only worth the trouble if you eliminate the passive crossover in the speakers. I hope you are planning to do this or it will be a lot of work for only modest gain.

This link explains more http://sound.westhost.com/bi-amp.htm

Like Jeff says, you need to be able to attenuate the more sensitive amp. An active crossover is always best. You could try these although I've never used them myself:

RCA line-level attenuator:

http://www.partsexpress.com/webpage.cfm?webpage_id=3&SO=2&&DID=7&CATID=48&ObjectGroup_ID=195&manuid=265

Or maybe a pair of the "Endler Attenuators":

http://mysite.verizon.net/vze4c5pt/id2.html

Dave

I use VTL MB100 for tweeters and Sunfire SRA at the bottom.
My preamp Sunfire SRP has two sets of outputs one is high-pass and another one is main. SS amp is connected to the main and tube amp is connected to high-pass through the balanced line converter with adjustable sensitivity.

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?
.

Works great with my Martin Logan SL-3's driving the woofer with a PS Audio GCC-100 & Cary SLAM-100 mono blocks driving the 'stat panels. The GCC-100 'gain cell' volume control drives both the power amp section of the GCC-100 and the Cary's via the output jacks on the GCC-100. I don't need to balance anything, the GCC-100 is 100 SS watts per & the Cary's in triode are 95 tube watts each - seems to be a great marriage for the Martin Logans - more than enough power & the tubes on the top really flesh out the panels, which can be a tad lean with SS amps.