Question about byamplification

@plga
To do it right, you need an external electronic crossover, like a Marchant crossover (see link below). You are going to have to disconnect at least the bass driver from the passive crossover. Not sure if your speakers are 2-way or 3. If 3-way, you can leave the tweeter and mid connected to the passive crossover. You going to have to find a way to deal with the gain level between the two amps. When you are done, your speakers will sound like they cost at least 5 times what you paid for them, I kid you not.  The point of the electronic crossover is that it will address the weakest link of your speakers, namely the passive crossover. 
https://www.marchandelec.com/xm66-electronic-crossover.html

Thanks for the answers.

Yes, that's a good idea, but for now I would like to know if I can try byamplifing with the passive crossover of the speakers without risking the tweeters or the amps.

The point of biamplification is to allow the amps concentrate on the frequencies specific to the load. So you need to cross over the signal before it gets to the amplifiers. You need the electronic crossover if you are to do this right. 

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.

you don't need external crossovers to biamp so long as the most powerful amp has gain control. Many will tell you that you do.  Those of you who think your internal crossovers should do the job are correct.   

As I interpret the OP question, and i don't know  the answer, since his speakers are 2 way, and the tube amp will just be powering the tweeter, so  the tube amp will sometimes have very little load on it.   can it be damaged from that (like a tube amp can damage transformers if running without a load on them). 

Jerry