How big did you do with the grid stopper? 1M is fine, won't cut treble (due to there being no voltage gain) and may help to smooth it out.
My take is that because there's no mid scoop / not much treble boost, there's not enough treble from the instrument for power amp harmonics to 'mingle in' with, causing them to stand out rather.
Putting in a 6G3 vol/tone mixer arrangement seems to help with this, and also as the 2nd stage may be exhibiting blocking distortion as stock .
Also the VB+ can be near 400V with some transformers / rectifiers, which can be good for a hard edged rock tone, but ~350-360V can sound sweeter.