I've been dual boxing (and for a little bit triple boxing) some of my chars just to get the EB badges and while my Ax/SR scrapper only had an issue once with a Crone draining my end, no biggie for them. The not very well built Thermal/Pain Defender got whacked a few times before I could pull aggro, but that was the defenders fault for using enflame. 😄
Oh right, the point:
From what I can tell, (and heck, may be in notes somewhere) is that the strength of the door spawns depends on population near it. Sorta like the invasion actually, but not sure. So, when I was off in a under populated area, I'd get small groups coming out for the most part. If I was around others, even if we were not on the main team, spawns were larger, and would be higher level.
I also noticed when we were off elsewhere, the spawns were quite random in size. Sometimes one EB, sometimes a three minions, sometimes six mobs with a mix of minions, lts, and boss.