Surely the best thing for getting the best tone out of your valve amp is an attenuator, not a pedal. The best distortion on a valve amp is that from the power amp, which means cranking the volume.
I have a tube amp, but the dist at the highest level is just not enough
I assume you're meaning cranking the gain up. By doing this your tone will become mushy & ill defined, keep the gain low to retain string definition, and crank the volume instead with an attenuator.
A TS-9/808 won't meet the "high gain" requirements, I tried a few a while back and didn't think they were all that gain-y, great for blues/rock though. Willo's mention of a booster pedal is a good suggestion for a valve amp, but whether this will give you the gain you're after I'm not sure. Saying all that, the ProCo RAT's have inordinate amounts of filth on tap. ;)