The question becomes whether the damage difference is enough. By my reckoning they are a net 40% down on the soldier (20% more likely to get hit and 20% less likely to hit) which is the closest class.
Soldiers may be the closest role concept-wise, but they're probably the strongest role, on average, because of their great AC/attack bonus. So I don't think Brutes need to make it all the way to a Soldier's level of power.
The main problem is that the damage doesn't factor ability score bonuses. So personally I would return Brute solos to using the 10 hp base.
For MM1 Paragon/Epic solos, returning Brutes to the 10 HP base but multiplying Paragon/Epic solo HP by x4 instead of x5 would leave them with -Con score HP compared to their current HP. That’s an easy change, numerically. However, if you did this, would you also want to change Solo Artillery/Lurker HP so it mirrors their 6 HP base? That’s not clear to me.
There’s a general problem with applying "one-size fits all" solutions to published monsters as WotC changes monster design. If WotC takes steps over time to compensate Brutes/Artillery/Lurkers solos for their adjusted HP, whether you'd want to still apply your original fix to the new solos that were designed with this in mind. Maybe when MM3 comes out they’ll have a podcast where they mention “we found out this was an issue and fixed it in this book” and then you’ll know to modify MM1/MM2 solos accordingly, but I doubt it.
Looking at the Bebilith and
Heroslayer Hydra preview (L20 solo brute) (I had forgotten that one), both have effectively greater attack bonuses than the Brute’s standard level +3 vs. AC. The Bebilith reduces enemy AC over the course of the encounter, and the Hydra attacks are at level+5 (the two eldest White Dragons in the MM1 have attacks at level+5 vs. AC as well). So maybe WotC is learning to compensate Solo Brutes better in general.
I don't think the Bebilith deals enough damage.
A Level 18 brute should deal 3d8+7, instead it deals 2d10+6, while its venomous bite deals 2d8+6 (and its a recharge 6 only usable while bloodied).
The Bebilith gets 4 uses of its claws a round, though, and the AC penalty is nasty, so it can deal less than the high damage expression per hit. By the way, I don’t like using the DMG damage expressions straight up, because they lump 3 levels together each time and have some very odd choices (why is it that the “high damage expression” is the same for level 16-18 and 19-21 while the medium and low-damage expressions both increase?) Fitting a line to the high damage expression suggests that level 18 should be 19.8 damage. Not a big difference, but it’s something to consider when designing and evaluating monsters.
I would have made the venomous bite deal 4d12+7.
I still don't think WotC is utilizing the limited damage expressions to their fullest extent.
I agree that Venemous Bite should deal more damage. A recharge on a 6 “only when bloodied” power should be better than its regular attack, but the Bebilith generally won’t want to use Venemous Bite if it has the opportunity to hit two foes with its claws.
The Bebilith's Flaming Web is a minor action, which is strange because the Bebilith doesn't receive any minor actions, but giving it one minor action between its two turns a round would significantly power up its offense.
The last issue with the Bebilith is that having it go at initiative counts 10 and 20 is too late in the round; 25 and 15 would be more appropriate. You don't want the PCs to all beat initiative 20 (which could easily happen with a Warlord with the Combat Commander feat) and have the Bebilith going twice in a row every round.
