About the scope, if you short probe tip to the ground lead you should of course see a fine trace with no noise. If you now touch that still shorted probe tip and ground wire to the correct ground point in the amp (the ground relevant to the part of circuitry you are measuring) then the trace should remain essentially unchanged. If it suddenly becomes noisy then there are some issues to resolve with the measurement set up.
X10 probes can be prone to noise pickup but that is more applicable to radiated interference from the likes of switching power supplies. If the scope is battery powered, then definitely try it in that mode with no external PSU attached.
I've not got the circuit in front of me but would suggest you are best comparing levels with the good channel to see where things start to fall down. Absolute signal levels are a total unknown and depend on the applied input level and any settings or volume level applied within the processing stages. Comparison is the best method I think.
X10 probes can be prone to noise pickup but that is more applicable to radiated interference from the likes of switching power supplies. If the scope is battery powered, then definitely try it in that mode with no external PSU attached.
I've not got the circuit in front of me but would suggest you are best comparing levels with the good channel to see where things start to fall down. Absolute signal levels are a total unknown and depend on the applied input level and any settings or volume level applied within the processing stages. Comparison is the best method I think.