i've never heard the 3a's, but aren't they quite efficient? may be a better match for the cary. proac recommends an arc tubed preamp & arc solid-state 100.2 amp to go w/proacs. i have heard this w/proac 2.5's & it does sound great.
in your situation, i'd suggest a pair of those 8wpc asl mono-blocs for sale here on a'gon for <$500/pair. these have a volume pot, so you could match them easily in your present set-up, in a bi-amped configuration, w/the asl's on your 1.3's tweeters... or, check the amp voltage outputs, you may get lucky & find that an asl wave monoblock, $238 per pair, & ~10wpc, may match up w/your mccormack amp, w/no attenuation needed. but, those *spendy* asl's have a much beefier transformer as well as the volume pots - prolly worth the extra money anyway...
http://cgi.audiogon.com/cgi-bin/cl.pl?ampstube&1015853800
http://cgi.audiogon.com/cgi-bin/cl.pl?ampstube&1016369148
doug s.