Change to Barclays car insurance, they gauruntee (sic..) to beat any price, and they have unlimited insurance on stereo equipment (!!), so says their website anyway.
Aparently they do alloy wheels and modifications etc without extra charge too..
Assuming they do, im moving to them when im next up for renewal :)

If you dont have the option to change car insurance companys, i think there are others that will insure car steros seperately, ask your local dealer or do a search..