not sure about what you mean about the headroom, but, you can run your power tubes hotter, which generally means they're going to break up earlier, which means a less clean power headroom, but a bit more wattage if you so choose to set it between 35/42ma (42ma is getting up around 70% dissipation on the plates of the output tubes).
depending on WHICH tubes you have, there could be a huge difference IMHO in tone based on cold/average/hot settings. i will say this, IME, there's a difference if you PLAY PRETTY LOUD, yes, it's more noticeable TONALLY as well as volume wise when you set the bias around 15ma(*cold as in near factory settings) and there's definitely an increase in volume and emphasis on tone becomes more apparent around 42ma(hot) when you get above 2 on the post gains. there's DEFINITELY a difference in feel between hot bias and cold bias, hot feels a bit mushier to me especially with sovtek 5881 wxt tubes. you should also notice a difference in the presence, it should sound a bit less harsh...at least my ears hear it that way at higher volumes.
for louder volume, personally, the way i play, and the style i play and want to feel/hear, i prefer colder or average....around 25ma per tube with 6l6/5881 in the 5150 amps for metal, drop d/ c tuned.
The 5150 generally run around 500volts at the output tube plates and setting them around 25ma is what i've found for a faster response and a more metally feel, and a more sterile metal push from the speakers, that's what i like (with k100 or t75 speakers).
now, i find when im running at lower volumes, i like the bias a bit higher especially when i run it through my cab with a v30 in it and im jamming ideas....it feels easier to play with a higher bias at lower volume settings when im jamming, and for some reason, feels more inspirational and easy to play...notes seem to fly off...but again, at higher volumes, i tweak it down, and like to work for my notes. go figure.
obviously, YMMV. there are things about a bias mod one person may say "THIS IS GRRRRRRREEEEEEEEEEEAT". ...and things about it that another may say "MEH...just let me plug in and play". most people will agree, with the 5150/5150ii amps, at with post gains above 2, you'll start to feel and hear different bias settings easier than trying to do it at bedroom volumes, or less than gig volumes.
it's an easy mod. the 5150ii already has a bias adjustment on the back of the amp, but a lot of people complain that the range doesnt offer enough sweep to get the tonal results desired (ie, higher bias settings). counter clockwise is colder, clockwise is hotter.
here's a good breakdown, and it's short.
http://www.sevenstring.org/forum/gear-e ... s-mod.html
