What are some of the things to know about biamping? I am considering this but do not know very much about it.
I need info on the active xover and what not.
Jsman, thanks for the info. please tell me what speakers you have and the approximate size of your room.

using an SET amp combined with a solid state amp will be tricky...but do-able. there are many that do take that approach.

personally, the approach i might take would be to use some sort of 'Y' splitter with one amp simply attenuated with a passive volume control such as the Placette RVC which will be transparent in the signal path. this would be much simpler than any active crossover design and with amps of that level you will hear it. an outboard crossover can yeild some advantages by not sending the entire bandwidth to the SET but IMHO at too great a price (both $$ and sonically).

there are impedence issues which are beyond my personal level of expertise that you could run into when using a splitter and a passive attenuator.

your amps are both quite special and if properly combined should sound great.

maybe someone with specific SET/SS biamping experience can chime in with other suggestions.
If anyone is interested I have drawn-up 6 illustrations of different bi-amping options. Some illustrations are with an active crossover. I really did it for my own understanding, but if you would like them I can send them to anyone. They were drawn in Autocad, but I will send them in .pdf format unless you have ACAD. Pdf's require adobe reader which is available free.
I use a Outlaw ICBM to combine SA220's (monoblocked) to woofers, mid/tweeters with a ML9. Seems to work fine.
You need a crossover to equalize the gain/volume on differant amps and set your crossover freq/slope.
Just gives you more toys to play with.
MikeL makes some great points. I use two EVC ultimate attenuators in my biamp setup.

But I think in this case it really makes sense to go for the active x-over since you really do want to filter out the bass freqs going to the 300b SET amp for the tweets since your available power is fairly limited.

That should increase your dynamic range allowing for higher max SPL.
If going for the active bi-amping the thing you need to keep in mind is the woofer x-over freq and the slopes of the high/low pass filters of the speaker's built-in x-over. What type of speakers are you planning to biamp?