I did some experiments and found the PFET switch has a limitation of about 1.6 volts total swing it can tolerate without clipping (using a 470 ohm load) you can get about 2 volts max with a higher load of 4.7K or so. This is perfectly fine for most guitar applications.For more headroom you can use a an analog switch such as the TS12A4515. A number of manufacturers make this device. It is a normally closed analog switch. Of course others will work also. Using this as a switch basically makes the supply rails the headroom limit. I am going to play with the circuit some more and come up with a board layout and will post my final design in a few weeks.