I would think with the nominal 4 ohm rating it must dip well below that amount in the bass frequencies. So, I would say it would not drive the Legacy very well.
Check with Legacy and see what the impedance curve is. As I recall is does have a dip in certain bass frequencies with the rest of the range being a nominal 4-ohm load. Bill and the team will get you the most accurate info. You should also get specifics from the amp manufacturer you are considering when you talk with Legacy.
I am running a 300wpc Mac MC302 amp into a pair of 93.5db efficient speakers with powered woofers (1800 watt amps built in) and the sound is full and big. When I had Mac MC152 150 wpc amp the sound was good but not as full. I would keep your amp. Even though the meters may read only 30 to 40 watts on peaks, the dynamic headroom is there for macro and micro dynamics. I think you would be going backwards by going with the smaller amp.
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.
This was a long time ago, but back then I had talked to Bill Dudleston about using tube amps with his speakers. His response was: "If you buy a tube amp, all you're getting is a box of trouble."
You can take this response with a grain of salt as it was several years ago. But just something to think about...