Question:  After reading the PIC-4704 user manual can I power the unit with a main station (say a MS-704) or do I need to use a power supply (PS-702 or PS-704 for example)?  Will a main station introduce the channel audio into the PIC-4704?
Answer:  It is perfectly ok to use either a Clear-Com main station such as the MS-702 or MS-704 or a Clear-Com power supply such as the PS-702 or PS-702.  The PIC-4704 IFB interface power input / loop thru connectors only use pin 1 (ground) and pin 2 (+30 Vdc) of the 3-pin XLR connectors. This prevents the audio normally found on pin 3 of the XLR from being introduced into the PIC-4704 interface. 
Audio  to the PIC-4704 is routed via the Program 1 and Program 2 XLR connectors, these connectors are looking for dry balanced line level audio.