Crossover - hard to find for biamping

I’m bi-amping with tube monoblocks into the midbass and tweeters using the passive crossovers, then running a solid state amp into the woofers. I’ve split the preamp signal so that one leg feeds an active low pass crossover to the woofer amp. It’s a very inexpensive Pyle PLXR2B for automotive use, but using a suitable 12V wall wart it works remarkably well. It has low pass options of 50, 63, 80, or 100 hz, and a separate gain.  Not much to lose at $30.

You only need to adjust the levels if your amps have different gain.  There's a convention but not a requirement for that.

If not, any passive volume attenuator will work. Put it before the loudest amp. :)

Wait

You shouldn't need a crossover if you are using the existing built in crossovers just volume controls.  It might get tricky if the amps have a different gain profile as you adjust for one global  volume level and then if you increase or decrease the volume the bass amp and the mid tweeter amp are no longer in sync.  It may not be much of an issue as you can always just increase one of the amps volume level to compensate.

If you're thinking of leaving the frequency dividing to the internal passive x-overs and using the active x-over just for level control you may not need one. If you choose tube and SS amps with the same gain (e.g. 26db) they should power the drivers to matching levels (even if they have different power ratings). Perhaps other, more technically minded members can verify my thinking. Then you would simply have to "Y" the L and R outputs of the preamp into the two sets of amps. Generally, the less electronics the signal has to pass through, the better.

I doubt you will get much value out of this configuration.  Active biamping can be very rewarding with an active crossover and bipassing the passive ones in the speakers. Leaving them in, well all you are doing is combining amps.