I'd like to know if my PG58-QTR is a low impedence mic or a high impedence mic. Is it suitable to plug into a guitar amp for practicing?
All PG models are low impedance.
The PG58-QTR is low impedance (300 ohms). It will not work into a guitar amp as the guitar amp requires a high impedance microphone.
To make the PG58-QTR work into a guitar amp, you will need a new cable (Shure model C25J) and a step-up transformer (Shure model A85F).