Quote from GarageYears; my inference added in square brackets:
"...it would be more likely that the combustion/power turbine stage [rather than the fan] would be the point where "wear" would introduce power loss, and hence imbalance the RPMs as a function of equalizing EPR."
If you are referring to the core of the engine, then I think you are right to point out that the higher pressure used for the engine pressure ratio (EPR) is sensed after the last LP turbine (I think!); not in the bypass duct or C-duct. Perhaps someone can confirm that?
In the line of my post you quoted, however, I was not talking about trying to equalise EPRs; I was talking about engines that use N1 as the primary indication of thrust:
"...in the case of what is sometimes loosely referred to as an 'N1 engine', such as GE and CFM, the thrust is considered to be a function of N1. EPR is not used. As the fan typically generates about 5/6 of the thrust, it's not a bad way of achieving the requirement, unless the fan of one of the engines is more worn than the other(s)."
In the case of engines that use EPR as the prime indicator, I argued that a worn engine would need a higher N1 than its brand-new neighbour to achieve the same EPR. On reflection, I think that both (or all three) shafts would be faster, as they interact with each other.
But the "low frequency vibration" and/or "beat frequency" that poorjohn notices is presumably from the N1 shafts.