The SP-14 has a circuit that detects wall voltage dips (and peaks too I think.) When/if that occurs, the preamp will switch back to "standby" mode and if you do nothing it will go back into "operate" mode after about 45 seconds. (It talks about this in the Owner's Manual.)
You could get a power regenerator/conditioner unit that maintains constant 120V, or put the preamp on its own circuit so it's not affected (as much) by other devices on the same dircuit turning on and off.
You could get a power regenerator/conditioner unit that maintains constant 120V, or put the preamp on its own circuit so it's not affected (as much) by other devices on the same dircuit turning on and off.