I guess external crossovers are for speakers without internal crossovers. Am I missing something?
Setting levels for two different amps.
plga--I used a similar amp to yours, the JAS Array 2.1, also using 300B and 805 tubes for several years. I used it in a biamp set up using Legacy Focus clones and it drove the midbass, midrange, and tweeters beautifully. I never ran it full range so have no idea of the bass quality but it reproduced the best mids I've ever heard in my system(s) and could float a most tangible 3D soundstage. However, it, and the other tubed amp(s) I had, the VAS Citation II, weren't the best at reproducing extended treble. Both amp nailed the critical midrange and had surprisingly dynamic mid bass but neither seemed to fully reproduce the top end. It seemed slightly truncated although never veiled. So, with that in mind, I must ask if running your Line Magnetic just to the tweeters might not loose much of that 805/300B magic that makes these amps so wonderful to listen to?
With regards to biamping using the LM and a Class D amp for the woofers, you can do it without an outboard/electronic crossover by having another volume control. For me, it was an integrated amp for the woofers so I could control their output and later a small Kingrex preamp driving a D-Sonic M3 600S. I would dial in the bass level via the Kingrex pre and control the system level using my DAC volume control. You could pick up a small preamp to do this as well, maybe even a headphone amp/preamp although the extra circuitry and interconnects might reduce the transparency of your system. After all, that's one of the major benefits of using SET amplifier.
I recently bypassed the crossover in my speakers to the woofers running them directly to the binding posts and using an electronic crossover, the NHT X1 and it was a noticeable step up plus the NHT X1 has a volume control on it so I pulled the extra preamp out of my system. Either way should let you biamp. You might not appreciate having the Class D amp reproducing the mids though. Best of luck with experiment.