I think the proposed amp will drive the Legacy's but the question is how well?
what has been said to date here is right on. the bass may not get the HP it seeks all the time.
Legacy sup will definitely put you on the right track, but do have all the bases covered about the proposed tube amp in hand when you reach Legacy.
JIC and FYI....
I've said it here recently I heard a Carver 75wpc stereo amp drive a pair of KEF Blade IIs pretty dog gone well in a medium sized room and thought the basss then was remarkable considering the 75w per $2500 amp doing the pushing. Really.
the Blade 2s are not as effective though they may not dip as low as the legacy either. dunno. and that part is the ticket here I'm sure. find out.
good luck!!