Biamp question

You can bypass the crossover in the speakers or you can leave it the way it is. The danger of disconnecting the internal crossover and running just copper wire between the low end and mid/high frequency drivers is the risk of damage to the midrange driver (the 7" in your case), should a power amp or the active crossover fail. You cannot bypass the upper midrange and tweeter drivers because that would mean you will need to tri or quad amp. I'm tri-amping with 3 stereo amps and a 3-way crossover, I also have 3 subs. The sound improvement with running just copper wire between the drivers and amplifier outputs, for me, is worth the risk of damage to any or all drivers. I recently lucked out during a very rare, (first time in 43 years) when the active crossover in one channel failed sending 500 watts into one woofer and 300 watts into one midrange for about 5 seconds. The tone was relatively high, about 900Hz., but any longer the drivers would have been damaged. Luckly, the tone was below the crossover frequency of the tweeter, so a clipped 200-watts rms signal was not seen by the tweeter! My suggestion is to take small steps. Leave the crossover in the speakers as is and as you get more educated and confident, you can try bypassing the internal passive crossovers one at a time. Enjoy!

I'd be careful with and active crossover that doesn't fail safe. A Behringer DCX2496 will fail to work but not send the wrong signal to the wrong place. I only use an active XO in my Bass management system. I find there is a lot better protection with a passive XO for mids and high. 300 hz and below I'm direct coupled too. No passive crossover. I don't recommend it, out of the gate though.. Hook up your amps and be happy. Quit making it complicated.. You want to mess around later, you always can.

Regards

If the output of the Parasound is louder than the Rouge then you are good. But if the Rouge plays louder, turning the Parasound down won’t work.

I’ve bi-amped with two of the SAME amp, but slightly different minor versions, and felt even that sounded a bit disjointed. So I’d be surprised if a high power SS amp and a high power tube amp meshed well. That said, the Rogue tube amp sounds much more akin to SS amps than you might expect for tubes. And the A21’s gain control is certainly an asset for gain matching the two amps. IF you find the Rogue’s gain to be higher than the A21+ at its max level, you can drop the Rogue down a few dB’s by subbing 12ay7 in the V1 / phase inverter slots in place of the 12ax7’s. It works well; I've tried this. I like the red-label GE 12ay7 from 1962 or 1964, I think (very specific year on the red labels) :)

A 5751 sub its also good for a dB or 2 and sounds nice on its own merits (if you have a good triple mica black plates vintage).

I'm doing exactly what you trying to do. I use a Parasound 21 for the woofers and a tube amp (set 300B's) for the tweeter and midrange. I looked at your preamp outputs. I run my Parasound with XLR outputs from streamer/Dac directly to the Parasound and  RCA outputs from streamer to my set amp. My streamer /Dac does not have an output selector so both outputs work at the same time. You will have to figure out if outputs from your preamp can be run simultaneously. Otherwise you will need a splitter. The gain from the XLR out will be twice what the RCA output is, so if your stuck using a splitter use XLR. Also, you are correct as the gain control on the Parasound can be used to set the woofer volume to match the tube amp output. I use a program called REW and microphone to adjust the output/volume levels on the Parasound to match the tube amp volume. But that's another "rabbit hole" No messing with the crossovers-Parasound to woofers and tubes to tweeter/midrange speaker posts. Sounds sweet. 

@backdoor 

"Thank you all! I believe I can manage the active crossover (possibly) but my chief concern is disabling the crossover in my speakers."

As it should be.  The crossovers do much more than dividing the frequencies.  They also may include-

1) Baffle step compensation (reduces front baffle reflected midrange and treble so as to even those frequencies with the bass, can be as much as 6db. 

2) Impedance flattening circuit.  Makes the woofers easier to drive and have a flatter response curve.

3) Notch / peak suppression filter.  Reduces the magnitude of driver break up frequencies or other frequency peaks that a typical crossover and slope cannot deal with.