I have done some research:
PiTFT - Assembled 320x240 2.8" TFT+Touchscreen for Raspberry Pi
When you compare these specs the conclusion will be that GPIO #18 cant be used on the PiTFT. This is a spot on the PiTFT for an OPTIONAL switch so that must be no problem.
But what bothers me is the power consumption!!
The question is which power rail is the PiTFT using and will it affect the DAC?
So when you have this setup please post your experience and confirm this combination is working (or not)?