Getting the most clean headroom from a 50 cal +

Well, it's time to change tubes on my amp (one of them just went microphonic tonight) and I was wondering what tube I could use to get a cleaner sound?

I heard about using a 12au7 in place of a 12ax7 in the preamp, and was wondering if anyone has tried that? Likewise, what about power amp tubes? I have a pair of mesa str420's in there. Are the new mesa 6l6 any better/worse/the same for headroom? What else could I try?

Past new tubes, any other tips/tricks for getting better cleans? I don't really use the lead channel on the amp anymore, so I'm not too concerned about how the distortion sounds. I just want clean and loud.

Do you use the reverb? The tube for the reverb is (or should be) a 12AT7, with 30% less gain. Try that. A 12AU7 has 80% less gain.
The newer Mesa STR-440s are very clean with lots of headroom.
In any case, running the Master as high as possible will give you the cleanest sound.

PREAMP TUBE GAIN FACTORS:
Tube Designation --- Gain Factor --- Can Be Substituted With
12AU7 ------------ 19 ----------- 12AV7
12AV7 ------------ 41 ----------- 12AY7
12AY7 ------------ 45 ----------- 12AV7, 12AT7, 5751
12AT7 ------------ 60 ----------- 12AY7, 5751
5751 ------------- 70 ----------- 12AY7, 12AT7, 12AX7
12AX7 ------------ 100 ---------- 12AY7, 12AT7, 5751

The 5751 tube does wonders for clean headroom. It has the same volume as the 12AX7 but 70% of the gain. There are preamp tubes with less gain than 5751, but those preamp tubes aren't as loud as a 5751 or a 12AX7.

Also, how warm are the EL84's running in that amp?

Don't put a 12AU7 in there, it takes the volume down noticeably.