Truely Balanced Pre Amps

Threshold T2, will still compete with other fully balanced preamps.

an excellent preamp. Only available used, of course.

A 'fully balanced' preamp will not have the signal appear anywhere in the circuit in single-ended mode, except at an input. So all the switching, the volume control, EQ is all in the balanced domain.

So how many drop out when this rule is applied?

The advantage is lower noise and lower distortion, to the point that less gain stages can be employed.

There is less than double the parts so the cost ought to be less than double too. And yes, there is a fully differential way to do this.