I just picked up another Runt 50 head, and bright would be the last word I'd use to describe it. Granted, I retubed it, (JJ KT77's, and a mix of preamp tubes), and it only gets bright if you get overly crazy with treble/presence. I play through a Randall 4x12 with broken-in UK-made Greenbacks, and actually run the presence quite high, (2 o'clock), and treble at noon. I find higher presence brings more hair to the gain, while keeping treble down tames the highs still.
Might be my signal-chain, but harsh certainly doesn't describe my tone. I have the boost engaged, gain dimed, and hit it with an Overdrive pedal - its one of the most pleasant, crushing tones I've ever had. I absolutely love it.
I didn't spend much time with it pre-retube, and I know Dave likes the EH EL34's, but to each their own.. I don't care for them, never have. I can say for sure, the KT77's added some punch.