A little suggestion I have : create a new research in specifically the Endless Research tree that would allow you to have more levels on the Balance Modifier.

As it stands, the max level for the Balance Modifier is 5, transferring xp at a rate of 100xp/s between towers. Mostly enough on Normal Mode, where max level of towers is 20 (unless specific bonuses such as Cores, the Bonus system of 6.6 or DQ, or well the Balance modifier), but feels way too little on Endless Mode, where max level of towers goes up to 150 without bonuses.

At the rate the Experience modifier can give xp on Endless (helped by an extended XP given research), it would be nice to allow its younger cousin to follow suit. 5 levels of research (to allow the modifier to reach level 10) should hopefully be enough, I trust you to figure out the values of xp transfer for these 5 new levels ^^