This is an amp that's been on my list for a while to check out, but they don't seem all that popular so mods are few and far between.
So I'm speaking speculatively instead of from direct experience here - but there is a highly detailed Alchemist service manual available online with a lot more information than we usually get for amps. It looks like the boost for Ch2 is just a simple switch that adds R38 and C26 to the signal path - which I think is just changing the brightness allowed through between tube gain stages 2 and 3. So it's not adding a "boost" in the sense of adding a boost in front of the amp would be, it's probably lowering the bright values to allow more low freqencies to pass through to the next gain stage. You could easily tweak those values but it's never going to add a bunch of gain as in, shredding solo kind of gain.
You could add a switch to go between clean and crunch but there's probably a volume difference between those two modes that would mean you'd either have to add a separate volume control or do some other trickery to make it usable. There are also multiple relays attached to that crunch switch - it's not just one signal path being changed, it's multiple spots. RLY1 does something with the V1B tube bias, cold clipper it looks like? Someone can correct me there if I'm wrong. RLY2 looks like it mixes in another gain stage V2A. You may need to add another relay and drill a hole for another volume pot to match the volumes across different volume levels, it usually isn't as easy as adding a fixed signal bleedoff to match the volumes because matching volumes will be different at lower DB's than higher DB's, if that makes sense.
You could use the boost switch to add my gain by perhaps adding some diodes or some similar pattern in there, but it might get complicated. The Marshall Mk III amps had a diode clipping stage between 2 & 3 and I think those sound awesome so maybe you could do something like that, using that switch to add some fixed clipping to the signal there - but it's not going to sound like a boost before stage 1, just to be clear.
Here's the link to the service manual by the way. And to give credit where it's due, I originally found it on EL34World:
https://totallyradguitars.com/s/Line_6_alchemist_sm.pdf
Cheers and good luck! Eventually I'll get my hands on one of these and maybe I'll have more ideas then.
