How to troubleshoot audio imbalance?

Question

Despite that the balance is set to neutral (middle) the right channel delivers clearly lower audio than the left channel for all I/O. I didn't try with headphones directly from the amp. The amp is Sony STR-DE515 and the speakers are B&W DM601. Is there a way to measure where the drop is?

Update

I switched the cables to see whether the fault was the speaker and it isn't. Switching the cables swiches the fault to the other speaker. So the fault is the amp or the cable(?)

Update 2

The troubleshooting leads to the conclusion that the amp is faulty. So how can an amp lose 75 % of a channel instead of everything?

Update 3

I handed the amp to an audio repair shop who said it was a circuit in the amp that was faulty so instead I got a new all analog amp, it doesn't have a remote control and it's only teo channels (no surround) but the two channels sound great (my new amp is a NAD).

Answer 1

Your question contains part of the answer: eliminate the speakers as a factor by trying headphones, or by connecting the current right speaker to the left channel and so on to see if the imbalance follows the speakers. If they're not the problem, do all inputs to the amp show the same effect? If so, it's very likely the amp.

If there's only one input and you can't evaluate that condition, I'd suggest moving up the chain and reversing the left and right inputs to the amp. If the imbalance switches sides, then it's the device feeding the amp. If that device is a preamp with inputs of its own, continue up the chain flipping sides until something pops out.