As far as i can see, i don't think any nerfing is needed -yet-. What seems overpowered in one map, is a total waste of money and certain doom on the other.

Personally havn't touched the Volcano yet, and don't think i will, only use i would see for them is when you need that little extra kick to take down a boss. but since my only true problem are the red dwarfs, i have no need for Volcano.

Neoman though it sounds fun to have 1-2DPS added to slow tower, but 1-2DPS won't make any difference on later waves when you can afford to max your slow tower.

Oh one thing comes to mind, a +1radius at 3rd or 4th upgrade of Boom tower. i like playing with the boom tower, but on those walled maps they can't be used at all. That one extra radius doesnt do anything on non walled maps, since there 'should' be a tower at the place of its increased range.