Biamping with very different amps

No not at all. That's something I have wanted to try for a long time. That's the idea a bass specific high current amp and a tube amp for the magic imagery and tone. 

Not crazy. But you will typically need a means of adjusting gain as between the separate amps to make them equal. Usually done at the active crossover stage.

If passive biamping, the JC 1 will provide about equal to the maximum output power of the CAD 572SE.
Active biamping is the way to go!

Biamping with very different amps

I’m considering biamping the HF/MF units of KEF Reference 107/2s with a pair of Cary CAD 572SE monoblocks and the LF with a pair of Parasound JC 1 monoblocks. Does that seem a crazy idea?

No not crazy, sweetness of the tubes for mids and highs, and control for the bass with the jc1’s.
If your passive biamping which it sound as what you are doing, both amps would have to be exactly the same gain, which is near impossible unless the same brand of amps.

But this is an easy fix, the louder of the two amps just needs a passive volume control on it’s input to bring it down to the same level as the other amp, once that is set, the main volume is then done by your main preamp or source volume

Easiest way is to get a Schiit Sys $49 passive volume control.
http://www.schiit.com/products/sys

Cheers George

You really need an active crossover to split the signal into high and low pass outputs to drive the bass and mid/high amps! An active crossover will allow you to set the roll-off slopes (not doeable with passive) and levels for the best sound! Been there, done that!