Hey Billy Penn, welcome to the depot
. So reading these posts I agree it's the input jack. And, as the input jack takes the most wear over the years, it's a common problem particularly on the early (older) V1 units (with top switch, as you have). The rear jack mod replaces the front and rear input jacks and also changes the rear jack to be inst (guitar) level, it's currently line level (as MJMP said, if you boost your guitar signal with something that puts out line level and plug it into the rear jack that should give you enough level, but this is a test scenario).
Easiest fix is to do (or get done, how are you with a soldering iron ??) the rear jack mod. You can buy the mod kit from MJMP here (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm), it's not a hard mod to do but if your not into a bit of soldering, any decent tech can do it.