I had a similar problem with a DSL 401, does it sound like whoosh or a roar coming from the reverb? I think it is normal on this series of amps, although it drove me crazy, so much so that I returned it... that and it blew the same capacitor twice in 10 days.
To my ears it sounded like only the output from the reverb was switched. The front end of the reverb was still active though, so that when you were playing on a channel without reverb then selected the channel with reverb, all of the signal still in the reverb unit would come spilling out. That's the way it sounded to me anyway.