Not sure if this is the right place to raise this question but Google hasn't helped.

I have a pair of JAMO Compact 90 speakers which are standing on the floor upright. I get distortion from one of them and have checked this is not a problem with the amp by switching channels. If I tilt the speaker back to about 45 degrees the distortion disappears and the speaker sounds OK. Returning it to the upright position the distortion reappears. There do not appear to be any loose connections so what should i be looking for as a source of the problem?

Thanks for any advice