The TS9 aren't really meant to improve sustain or compress the sound at all, though they do improve the sustain quite a bit.

I used them a lot to boost my tube amp a bit before I switched the the DOD250. Think of it as just kind of hot rodding your gain a little bit, nothing tremendous but a great sounding helpful boost (very natural sounding).

I'd say as far as mids and highs go, its fairly even across any range of frewuencies.