The RV25 is a new model. Shouldn't that still be under warranty? :?:
Maybe you are far from a warranty station or the factory and want to avoid shipping...can't say I blame you.
If you can duplicate the problem consistently enough, you can try pulling the preamp tubes, one at a time, starting with first tube in signal chain (usually V1) to see when the problem stops. That way you will know at about what stage the issue is occurring. A bad solder joint, for example, will cause noise like that.
Of course, you'd still need to get it to a tech to repair.
Also, have you tried a jumper cable in the FX loops jacks? If the noise stops, you may have a bad jack. Mesa generally uses a switching jack for the return and if it is dirty, corroded, or just not making good contact, noise can be the result. A jumper will eliminate that as a possibility.
Good luck with that. I know the feeling of buying a new amp with a defect...no fun! :wink:
PS. I too have some old bugle boys...sound good, don't they?