SETs vs. Push/Pull

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".

You are mating the SET to the wrong speakers.

Look at the Canary line of 300B p-p amps. I'm using a CA-339 right now on a pair ot Coincident Total Victories, and it's a much better amp to my ears than the SETs I've been using, including the Wavelength Triton Blue, the deHavilland Aries 845G and a KR 18 BSI. There's no loss of musicality, but big gains in transparency, resolution and dynamics. Their smaller (and cheaper) CA-330 probably would have done just as well.

I tend to agree with Edle. I think you need to keep the amp / passive preamp (etc) and consider different speakers.