My PA was my monitor. I used the Bose L1 PAS and the Bose L1 Compacts, which made it possible for me to place the speakers to the side and a little behind me, just two feet away from the keyboard and hear exactly what my audiences heard, yet I never experienced feedback. Of course, the mics I used were very high end and feedback suppressed, such as the Crown CM-311A and the Sennheiser E-855. No external monitor needed using these systems, but year prior to using Bose, I used a dozen other systems, all requiring an external monitor, which was a 12-inch floor monitor made by Yamaha. I no longer have to pack those kind of things around.
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)