Best Midwoofers/Subs for Acoustat 2+2 electrostats

Need some help/expertise. I have been out of high end 2 channel audio for a while and want to get back in. My speakers, the Accoustat 2+2s, are still sitting in my room with a 15" powered HSU sub. I also have a beautiful symmetry crossover. I like the enjoyment of rich concert level sound. What I'd like to find are a pair of sub/midwoofers that play cleanly up to 4-600Hz which I can then cross over to the Acoustats from that frequency range. I'm not so much concerned about the lower octaves below 80Hz which the HSU 15" sub can handle. I'm looking for very quick, musical midbass subs that hopefully blend well with my Acoustat 2+2s ... Thanks all :)