Pure Essence upgrades pure towers and increases their DPS by 15x. Something similar could be applied to Duals and Triples. Alternatively, you could modify abilities to make them ~15x stronger.
For instance, the Blacksmith tower could give 270% Damage increase (3x lvl 3 benifit) and increase the AoE of the tower to (no mare than) 150/300 (this multiplies tower DPS by 3x if it was single target, 2x if it was 100/200, and not at all [i.e. 1x] for towers already with 150/300). This potentially increases the DPS of 4 towers by (270%)x3 = 810% or 8.1x for previously single target towers, 4.05x for previously small AoE, and only 3x for the previously large AoE. The rational here is that level increases (lvl 1 to 2 and 2 to 3) give 5x DPS but lvl 3 to 4 gives 15x; the Blacksmith Tower goes from .1 to .3 to .9, all increases of 3x for a "5x" DPS jump; therefore, the "15x" jump should be 3x more than the "5x" jump, so we need 3x3x0.9=8.1 total multiplier from lvl 4 Blacksmith. I guess you could add this all to the DPS increase of its ability, but I thought it might be fun to add the AoE in there since it would make some towers really strong in conjunction with slows but wouldn't benefit already-AoE towers much AND you would need to manually cast it to get the biggest benefit (adding some more micro the player needs to use. Just an idea).
Same thing with Well Tower but maybe increase tower range (need to do more calculations to figure out how range multiplies DPS) in addition to BAT.
Thoughts on other towers?
EDIT: Just thought about this buff on other Pure Essence'ed towers and that might be pretty imbalanced... That's why I'm not the developer :-X