My amp is producing a fairly noticable amount of hiss, could this me related to my guitar setup as it gets worse the more I increase the volume on the neck pickup rather than on the bridge pickup?

Anything to check on the amp? How do I know when the valve in the preamp needs replacing, should it be done periodically or do you just use it until it breaks?? Sorry if thats a dumb question but it is my first valve amp.

Cheers

Matt