Have you heard back from Line6 yet? I am curious to hear what they have to say from a technical standpoint about this.
I do think, however, that since you are designing a speaker box for these amps, that you should experiment with lower wattage speakers than what the amps put out. I have found that going with a lower wattage speaker can color your sound in interesting ways, that aren't always unfavorable. Of course, it all depends on what sounds good to you. A lot of people actually use a total speaker wattage of half of the amp output.
I do think, however, that since you are designing a speaker box for these amps, that you should experiment with lower wattage speakers than what the amps put out. I have found that going with a lower wattage speaker can color your sound in interesting ways, that aren't always unfavorable. Of course, it all depends on what sounds good to you. A lot of people actually use a total speaker wattage of half of the amp output.