When to bump difficulty?

So, I've rolled up a mind/stone who is a single target *BEAST*. Domination+PowerBoost = nothing going unheld (so far )

What level do you doms who mostly solo crank the difficulty? What settings do you go with?

I solo slightly more than half the time and only rarely in large teams.

I normally set my difficulty at +0/x1/+b as soon as I pass a Fortunata and leave it like that until I get DO level enhancements when I would increase it to +1/x1/+b. At SO level enhancements I will either increase levels or equivalency depending on available AoE.

For a Fire/Earth, you can increase your levels to +2/x1/+b as soon as you get Seismic Smash and potentially even higher depending on how you have slotted accuracy. You may wish to experiment with equivalency once you have slotted Fire Imps, but your AoE is not your Assault set's strength.

Personally, with the Fire/Earth I'm building at the moment I'm going to muck around with damage procs in Fire Cages and Hot Feet as an AoE damage mechanism and I intend to work on equivalency in the 20s to get a feel for that.

I haven't made any new doms since the difficulty options changed but I've tended to raise difficulty in the low to mid teens to rugged then so +1/x1/+b - some have gone higher - my Mind/Psi went to relentless (+2/x1/+b) about that point. Had to lower it for teams as the others couldn't handle it. I did clear an 8 man relentless mish at about lvl 18 but that was very slow going after the others gave up.
My Ice/En was just silly - came out of Breakout and attacked the first mob I came to before checking the level - they were lvl 6 mostly Lts and went down easily enough so I put the diff there to relentless before even hitting a second mob.
My fire thorns I raised the team size - so +0/x2/+b about lvl 10 when solo but that was built for a team primarily and really needed someone with a taunt to get the best out of her

Generally I use the rule of thumb that when the mobs drop easily its time to consider raising difficulty.
AoE heavy toons get mob size raised first, ST get level raised first.

Currently doing -1/8, 0/8 and +1/8 with no bosses on my level 9 plant/fire.