You want to bleed *less* to ground, making the bias voltage more negative. It's a simple voltage divider comprised of the 1k and 27k resistors. https://learn.sparkfun.com/tutorials/voltage-dividers
My two cents, I'd replace the bias supply with the one from the 6G6A; you could even replace the 10k resistor in that with a pot. Might be unacceptable for an amp this old though, but maybe start with replacing the 1k with a 470R.
Whoops, I had it backwards too; thought the raw supply was 59V and that he wanted to cool the bias off from 50V to 55V.