My DSL401 has just started to intermittently volume drop i.e. the volume gently fades down to nothing - leave it a while, and the volume returns to normal. Also, the effects loop doesn't seem to work as it used to - for example, if I plug a delay in to it, the delay is hardly noticeable when you play. Not sure if the issues are related or not. I've not had any issues at all over the last 12 years and the tubes have not been swapped out. Grateful for any advice. Thanks in advance.