If I do a 1/4" Y split off the main out of a mixer - one to my interface and one to the bands PA - will that mess up the signal in any way to the PA?? (or the interface)

Can't say for sure without knowing the impedances, but most likely you'll have no problems - assuming a short run, quality cable, and a common ground. If you do have issues, there are signal splitter devices for just this sort of thing.

Hey thanks !!!

Most mixers have a "rec out" , you could use that too.

You may get some earth grounding hum. Here is a relatively new device which is designed to solve the problem if that is indeed the case.

http://www.sweetwater.com/store/detail/DTI

It is so new it is not even on the ART website yet as far as I can see. It has got RCA, Jack and XLR inputs and RCA, Jack and XLR outputs with high quality isolation transofrmers between input and output. This would be a worthy device to have in any case.

Impedances may effect you as well but if the output impedance is low from the mixer then it should not be an issue, earth loop hum might well be your enemy in this situation.

I've got the Rolls Buzz Off, which is similar to the ART unit, except no XLR. It saved my bacon a couple of times in live sound situations. I've used them to fix ground/hum problems between the sound desk and the amps, and between the computer I/O and the console. A couple of these (or similar) are a must-have in the toolbox IMO. http://www.rolls.com/product.php?pid=HE18