Building a simple monitor controller. Sum to mono at beginning or end?

[JW]

I'm building a simple monitor controller. It's a two stage DOA design (API 528/536 line stage) with the fader inbetween the two stages. It has the DOA (990C is what I'm shooting for) as the balanced receiver, and then your usual API driver + transformer stage. Is there a preference as to where to put a passive sum to mono switch? Input or output? I can't see how it would matter one way or the other, but thought I'd ask. Maybe it does.