which ground reference for +12vDC?

[tempus fugit]

Hey all;

I'm trying to use an analog switch (MAX4661) to switch 12v from my guitar
amp to ground. This is going to be setup (actually it will replace a relay)
in an existing design which is functioning well. The current design has
12vDC available. However, measuring the signal from my amp I get about
12.5VDC. The 4661 datasheet states that the input voltage can't exceed the
supply (no surprise there). The 12.5VDC that I get from the amp naturally
uses the amp's ground as a reference. The 4661 will be using a different
reference point for its ground. So the question is......


Will the 4661 "see" the 12.5VDC as 12.5VDC or will it be some other value,
since the ground reference is different? Should I be taking the voltage
measurement between the hot signal line and the 4661's ground to see what it
will give? It'd be nice not having to rework things in the existing design
if I could avoid it, but I don't think I should put 12.5VDC on a part that
is using only 12VDC as its supply.

Thanks