The Mesa Boogie 2x12" Recto is closed back, and I tried the Black Shadow through my friend's 1x12" combo, so not the same cabinet.
I also tried an Orange HP cabinet, loaded with Celestion K100's, which I hoped would solve this issue, but this was a bit fizzy too (possibly in a higher frequency range?).
Is there anything I can do in relation to the tubes, that could solve this problem?