A 45 SET amplifier will work with Cornwalls, and will get average listening levels...but that all depends on the size of the room.
If it is large to huge listening room, then the 1.5 watt 45 could be a bit on the light side for power.
If the room is small, and if one listens primarily below 85 decibels, then a single-ended 45, 2A3, or even a SEP 6BQ5 would give moderate to loud levels. You won't ever get a continuous 105-115 decibels, but I doubt you are interested in SPL numbers when you want to build a SE 45 amplifier.
The 2.5 watt 2A3 will net a extra watt of power, but I never noticed much differences (slight) in power between 45 and 2A3...just tonality the most.
A single ended pentode connected 6BQ5/EL-84 amplifier can sound very nice with Cornwalls, at least to me with mine.
I've plowed a many single-ended 6BQ5 amplifier and amplifier circuits through my Cornwalls...Motorola, RCA, Magnavox console pulls, and the RH84 circuit. (which was eventually a completed amp) They can sound really good.
If it is a large to huge room, then a push-pull 6BQ5 amp with about 7-8 watts per side would work pretty good.(like Chris A mentions) My Scott LK-48B has plenty of boogie factor with good sound.
Mike
My room is 26'x17'x8'. Thanks for the info. that gives me some ideas. Very helpful forum.
Ritchie