SETs vs. Push/Pull

how about trying paul speltz's zero-autoformer first? relatively speaking, the plain zeros are cheap (about $500) compared to swapping amps, and you may discover you don't have to part with your beloved air-tight or proacs.
just a thought

How about just biting the bullet and get a transistor amp for those times when mood calls for fast transients/power/rock & roll?

Since you love triodes, consider a triode-wired PP amp such as as the c-j MV55 or MV60 (about 25 and 30WPC, I believe, from a pair of EL34s per channel), or a Premier Eleven-XS, again about 30WPC and EL34s, but this time with Premier-quality design and parts. And of course there are lots more brands around.

Try a higher powered SET amplifier such as the Antique Sound Lab 805 at 2995 new; 50 watts.
http://show.audiogon.com/cgi-bin/shm.pl?ampstube&958923080&item&Brooklynaudio&4&5&6&http://cgi.audiogon.com/cgi-bin/fs_srch.plQQANYAAAApurlsrchAAEXYAAst26AAAAAAantique_sound

Or an 845 parallel monoblock ASL..more expensive..
http://show.audiogon.com/cgi-bin/shm.pl?ampstube&958923923&item&Brooklynaudio&4&5&6&http://cgi.audiogon.com/cgi-bin/fs_srch.plQQANYAAAApurlsrchAAEXYAAst26AAAAAAantique_sound

The Mastersound is probably the best bet as it can be had for about 3000 used and is a lovely piece and has plenty of power for what you want.

A quality tube amp will produce fast transients and rolls with the right speakers. Tube amps with average power supplies have issues with fast transients.

All great ideas and I think the jist is to get a 2nd amp and use each when the mood strikes. The ProAcs are an all around easy load and I think I can experiment with just about anything if having two "drive engines".