i reiterate my opposition to the "constant volume" balance control unless it's a user switchable option.
The point is that to have constant volume when changing balance, one channel has to be increased and the other channel decreased the same amount. This means that if the attenuator has descrete steps (as in the Tortuga), then the minimum amount of change is two steps... one channel up one step and the other channel down one step.
When using the balance control to compensate for a L/R volume difference due to speaker, amp, room, source, or recording imbalances, the smaller the available change the better. So, for a constant volume shift, you have to change the net L/R balance two steps, twice as coarse as the channel attenuator steps.