It's a well known fact from mesa guys, rectoverb 50 and solo 50 (both series I & II) have a gap at switching.
When I asked mesa techs, they said that was done on purpose to avoid pops when switching...and there is np fix for that !
That was the main reason why I sold my solo head 50 (that I loved, huge sound) beccause as I'm a singer, I always switch on the first beat of a Bar and I hated that the disto sound didn't come quick on the step.
regards