Biamp question

You’re fine OP. You don’t need anything BUT two amps and cables. You have a good idea. SS for the bass and Valves for mids and highs. The Parasound will have the greater gain. It is a plug and play thing. It will take more time to route the cables than anything else. Remove the jumpers between lows and highs on the speakers and plug in. Adjust the Parasound. DONE.

Side Note:

You need two of the same for vertical bi-amp. You're using them horizontal. I've biamped for 40 years horizontal, NEVER vertical.
I quit using Valves for bass 40 years ago too. Just a bad idea in reproduction, not to mention the heat.
Only one time I didn't bi-amp and I fixed that soon enough. BIG fan of bi-amping with SS and tubes. 

I forgot, Merry Christmas and Enjoy...

Regards

Oldhvymec thanks for your advice and input. I will give that a try. Thanks to everyone else who commented for your input as well. This has all been quite illuminating. 

I do have one last question. I am assuming that both amps inputs go to the same preamps outputs. Is that a correct assumption?

You do not need a crossover- just let the speaker do the work. 

connect the bass amp to the bottom speaker terminals and the mid/treble amp to the top set of terminals. 

you may have a gain issue that causes a miss-match- 

You would need an RCA Y-cable to spilt the preamp signal.  

PS I have owned the a21+ and the Stereo 100.  Have you tried just the stereo 100?  I found it as powerful as the a21+ and your speakers are very efficient.  Try just the ST 100 if you haven't.  

Yes just split a full signal out to your preamp to you SS and Valve power amps. RCA or XLR. What ever you use.

Enjoy..