Ashdown head prob

[LITTLEWING]

My Ashdown EVO 2 300 head sometimes has a wobbly when playing a passive bass thru the active channel (simply to clean things up and use more headroom in the input). It sounds just like a speaker very faintly crackling starting to give imminent warning of dying although there's nothing wrong with the speakers, (it's an 8 x 10), as #1 I've put it thru another cab and it still does it, and #2 if I swap to the passive channel everything's totally fine. I know the active channel socket simply has a filter, so there's nothing wrong amp-wise. I'm thinking either dry joint or dirty/tarnished socket ring/tongue?

[Machines]

Drop Ashdown an email and see what they say: http://www.ashdownmusic.com/tech/tech_service.asp