Quad L subwoofer, plodding sound

I bought a Quad L subwoofer a few months ago and recently ive noticed a 'ploddying' sound when i go above a certain level (which isnt really that high). At this point i hear the 'plod' where i think the cone hits the voice coil and there is (very) limited punch in the bass at the point. When i first got it i must have stupidly put the volume on too high and damaged it? its very frustrating when i want my music on high levels. (i know now not to put the sub anything above 80%). The bass used to be much more powerful.
do you think its the voice coil thats broken and do you think its easy to repair? it comes with a 1 years warantee as i bought it from SSAV, the journey is about 40 mins away however and it weighs a tonne!
thanks in advance

Haz

[Crustyloafer]

Get it into the shop you bought it from, they will handle getting it back to the manufacturer for repair. The manufacturer will also assess whether the product has been misused and will or will not charge accordingly.