I'm thinking of buying one of these walkabout heads. It's a older red faced model. I hear these heads have problems with the effect loop. Are they a serial loop? This head you supposedly have to have a through cable connecting the send to receive to complete the circuit.
It is a series loop, but it is normally bypassed unless you plug a cable into one of the jacks. The problem that happens is that the jacks can get dirty/oxidized/bent in a way that bridges the gaps and makes the amp act like there is a cable plugged into the loop when there isn't, thus cutting out your signal. The "fix" that Mesa implemented on the new ones was just the addition of an effects bypass switch that pulls the loop jacks completely out of the circuit so they can't cause this interference. On the older models, a fix is either to leave a cable plugged between the loop send/return jacks or just to clean them or bend them back into place. It seems like there are plenty of these amps running fine without any issues with the jacks at all ( this has been a recurring issue on many different amps over the years- GK 800RB for instance), and its no biggie to fix if necessary!
The other upgrade was an Active/Passive switch vs. the 2 seperate input jacks on the older ones. I think the reasoning here was the same- dirt or oxidation from the unused jack could cause unnecessary noise in the signal.
D) with the rubber jacket in position 2, and I'm in tone bliss! 
