My "guess" is that this is not a low powered tube friendly speaker. Others with more direct experience with these speakers will have to confirm or deny that statement though. Sean
I meant to mention that there are many threads on the Apogee forum about bi-amping ribbon speakers. For some enlightening reading, you may want to do a search on bi-amp and bi-amping when you get there.
Since youre a tube fan, you may also want to do a search for Llano on this forum. The Llano Trinity is an interesting hybrid amplifier many Apogee owners like. Also, take a look at Llanos site for additional info:
Good luck with your system!