tubbsbass

Hi folks, new to this and many thanks to BD02 for steering me in the right direction! I use an Ampeg SVT3PRO with classic 4x10 1x15 cabs, Musicman Status Graphite 5 string, sounds awsome! But lately i have been getting a nasty crackling noise coming out as you turn up main volume, even when mute is on. Any other SVT users had similar problem? The amp is only 3 years old but has been well gigged! I don't really want to replace all the valves if it turns out to be something silly! Based in Bedfordshire and don't know of any good amp techs around here! CHEERS.