I just added a TS9 to my Dual rect... Not sure if it's the best pedal out there, but it seemed fairly mild and i didn't want to spend $180 on the Tubescreamer 808.
Anyway i run with El34's and this really made a small but noticable difference to my ear. The bottom end is a little less muddy, it does cut a bit of the bass out. I can't place my finger on it exactly, it's just one of those things, i plugged it in and it was instatly a bit better, not a ton but enough to warrant a purchase... just the tubescreamer magic i guess.
One thing it seemed to me was it made the amp sound "hotter", like it was cranked at even managable volumes, and cut through alot better... also added a bit of fluid and tightness to the sound. I played a stiletto Ace combo and the Fluid drive channel really shinned. I'd say with El34's and a Tubescreamer it's very fluid/tight just like that... still a rectifier, but very different than a stock 6l6 with mid scooped.
I'm also running on the raw channel now, with a bit of gain(about 10 o clock) on the tubescreamer. It's plenty of gain and really made that mode shine, the vintage is just high gain 80's marshallish now, i don't touch the modern settings.
Worth trying imho!