emitter follower oscillation cured by adding base input resistor?
Yes, adding a base resistor (usually a 50 - 1000\$\Omega\$) is one solution to RF oscillation of an emitter follower caused by capacitive loading. It doesn't take much capacitance, tens or hundreds of pF can be enough, which is not much length of cable.  

Often a ferrite bead is used, either in the base or emitter lead because it doesn't degrade the high frequency performance as much as a resistor. You don't need it for audio. 

See also Brian Drummond's answer here 

Your actual question is "what else could it be?". I think it's the most likely cause, and I can't think of anything else that would be improved by the 'base stopper' resistor.