The symptom is that one speaker sounds louder, now what we have to do is find the cause; find out why.
Before you swap speakers make sure your speaker wire ends are in good shape and that there are no stray strands of wire sticking out. Make sure they are cleanly and solidly place in their respective connectors.
Once you have absolute confidence in the wire, swap the speaker wires to the speaker is on the opposite channels. That should help you determine whether it is the Amp or the Speaker.
Also, if you have a Mono switch on your Amp, switch it to Mono while you listen for a while. IF the sound is roughly the same in Mono, that would indicate the problem is with the signal source (DVD, CD, Phono, Tape, etc...). Sadly most new amps do not have a mono switch.
If you swap speaker and the problem stays on the same channel. Then you have to do the same thing with you signal source. If you are playing a music CD, then you have to swap the output of the CD. Left channel to right, right channel to left.
And just to save yourself a lot of headaches, make sure the balance control is set to center. It could be as simple as someone messing with the controls.
By swapping things around (one at a time), you should be able to isolate the problem.