Okay, went back and actually read the only thing I could find on the subject, back here. Statesman said:
AND now…we’re changing the way Archvillains spawn. A ton of forum goers disliked adding so many AV’s into missions a while back…so we’ve come up with a solution. If the team size and mission difficulty are ABOVE a certain level, an Archvillain spawns. Below that, players will face only an Elite Boss. If the mission is set on the first two levels of difficulty, it takes 4 heroes or more to spawn an Arch Villain. On the third level, 3 heroes or more. On the fourth level, 2 heroes. On the highest (Invincible), a solo hero will spawn an AV.
So, for PC/A, we should be able to still get Elite Bosses at the 3rd level of Reputation — which should get us better (tougher) spawns, but still keep it in the duoable Elite Boss range.
We’ll have to give that a try.
Good guide to reputation here.
The third level is:
Rugged/Vicious – Mission spawns at +1 level to your level (i.e. +0 to +2 mobs, but usually mostly +1 with some +2). Spawn sizes are normal. AVs become Elite Bosses when team-size is two or less.
And this worked great — we did a (non-AV/EB) mish and, carving through orange and reds, the XP was faboo — about 14-15K/minute. Keep that up, and we could be at 50 in 3 hours or less.
As Margie noted, we need to consider what we want our Big Mish to be.