Whats the difference?

I just read 128 pages that came up on a search about biasing. The only thing I found is, "hot will shorten tube life".

I just replaced all my tubes. I put 4 Tung-Sol's 12AX7's in the preamp and 4 JJ's EL34's in the power amp. I biased it myself to 90mv. That is Marshall's spec for my amp. I thought I read somewhere ppl like to run them lower.

My question is what would I get from running it hotter or colder then spec? Would I get more/less headroom or gain, change in tone?

Hot= earlier breakup, better OD, up to a point. Crazy hot bias doesn't sound any better and will chew up your power tubes very quickly.
Cold=later breakup, fewer harmonics. Not always a bad thing, but most people prefer it higher.

If you biased it yourself, you can always experiment. Marshall says 90, but you can run it up and down a decent range and see what you like, as long as it's not too hot. 90-95 would be just about right for that amp, I think.