552fl,
I just finished making some repairs/adjustments to a friends Kris 600SSB. It also is a 1 driving 3 box however the driver is a 6JG6A (maybe a little less output than your 6LQ6 driver??). I was seeing 325 max watts on SSB with about 10 watts input. If you are planning to drive your SB-220 with something like that, it might be a bit stiff. If the Thunderbolt has a "Low Power" side, it might be worthwhile to just use that setting as a driver.
This, of course, is just my opinion. Take it for what it is worth. Good Luck.
73's
David