the power needed to drive speakers properly is very subjective, especially when using tube amps.
I have had power amplifiers of all power sizes, from 2 watts to 400 watts output, connected mostly to either normally efficient (~89-91 db) to low efficiency (~85-88db), and have never really had a problem, it depends on the type of music, how loud you like to listen, etc.
My former neighbor had ProAc 2.5s, and he first ran them with Audio Research 150 wpc tube amps. I loaned him my VTA70 for a week and he was wowed that my $500 amp sounded as good as his $3000 amp. Later I loaned him my DIY ST35 and he loved it so much he wanted to buy it from me.
He and I both listen mostly to jazz, at a medium volume.
The ST35 isn't quite enough for higher volumes. The VTA70 was fine except at ear splitting volumes.
I've also run 2w tube amps with jazz at normal volumes, but my VTA70 does make a significant difference in sound quality and power reserves. Back in the early 80s I ran 200wpc SS into JBL studio monitors, which was OK, but I can tell you 100w of tube sounded better. 350wpc into JBL monitors was astonishing, even more so because I couldn't blow the JBLs up, even though I could hear the drivers exceeding their cone travel limits in each direction (can you imagine the Doubie Brothers at FULL volume with the VU meters peaking at +3db above 350 watts!!
At any rate, I consider my 40wpc VTA70 adequate for MOST listeners, but again, if you like ROCK or symphony music EXTREMELY loud into ProAcs, then go for the VTA120 amp from Bob.
Now, the VTA circuit was adapted from Vacuum Tube Logic circuits back in the mid-80s specifically to fit the Dyna ST70. I've had 20 years to play around with the topology, including trying CCS on the bottom of the LTP, with NO improvement. Other circuits can design for no coupling caps, but at the expense of comprimises in operating points of the circuit. The better solution is just to use a great design with good components, that all work together with a synergy that makes great music come thru. Premium caps can make a difference, as can premium tubes.
Lastly, excuse any spelling errors above, I'm pretty tired today!!