Well, of course it's easy enough to swap cables at the amp and find out if it moves with the cables or stays off the left channel.

My next step would be to swap the speaker leads coming out of the amp. If it follows the amp leads, it's the amp. If it stays on the left side, it could be the speaker itself.

Obvious stuff I guess, just make sure you eliminate all other possible causes. However, I can easily believe it's the amp. I have had two of these in the past. One was fixed by Phoenix Gold - it was actually out of warranty but they didn't ask - I mailed it to them... The other I paid to fix (Rockford Fosgate, also out of warranty) at a local electronics shop (Not sure what RF would have done but I didn't try).

Good luck, may it only be a loose connection!