It could be a worn volume/balance pot.I have an 840BX4 with the same problem.A spray of cleaner lubricant into the pot fixes it for a while.
These are great little amps.I am not sure about the difference between the BX4 and the BX3 but I have been really shocked by how good the BX4 can sound.They are deserving of the very best matching components.
People talk about the Pioneer A400 being capable of very high end performance but I think these Rotels are probably even better.
I have been told by a long time Rotel dealer that these were Rotels best sounding amplifiers and that they are a very high current design capable of driving a 2 ohm load.