Hello tiofelon,
It could be a lot of things, from a bad source, amp or preamp channel or jack, bad interconnect or speaker cable, blown speaker driver and even a carbon or other compound build up on a preamp control.
A good approach is to try and narrow down the cause of the issue by a process of elimination or ruling out possible causes of the problem in a methodical manner, beginning with the end of the signal chain and working backwards or upstream. Here's how it works:
1. Begin by connecting the right speaker wire coming from your amp to the left speaker and the left speaker wire coming from your amp to the right speaker. If the problem follows the switch ( the opposite speaker now is lower in volume) then the speakers are okay but a speaker cable, or another issue further upstream, is the likely culprit.
2. If a new speaker wire or pair of wires don't solve your problem, then the problem likely lies upstream. You can test this by connecting the left interconnect cable coming from your preamp's output to your amp's right channel input and the right ic cable coming from your preamp's output to your amp's left channel input. If the problem follows the switch (the opposite speaker now is lower in volume) the ics are okay but a preamp or amp channel or jack may be faulty.
This process can help you isolate the issue or at least rule out some common sources of the problem. Of course, this process may ultimately require you to bring your preamp, amp or a source component in for inspection and possible repairs.
Best wishes,
Tim