I've been eyeing up the HSU STF-1 sub for some time now. I bought a pair of M60's for Christmas and I'm starting to save for a sub. I'm considering an STF-1 or STF-2. Ratio music to ht is probably 80:20.
So it sounds like you are happy with your STF-1. What size room do you have it in? What main speakers are you using it with? I read on the HSU board that some of the HSU engineers themselves were pretty impressed with the output of the STF-1.