Quick update: I have tracked down an amp tech called Dave (never asked his last name!), through some player friends. One of my friends (Mike Walker) has had work done to his Boogie by this guy and would recommend him.
I've spoken to Dave on the phone and he really seems to know what he's talking about. He says he's an authorised Boogie Tech, but I don't know how to check that, but I do quite a lot of my own work on my vintage Fenders, so I know when someone is blowing smoke up my ***. We talked about the FX loop mod and whilst he didn't have specific details he did talk about modifications which could be made. In the end he recommended that I have my amp fully serviced and recapped and then use it for a while. He said his standard procedure on an amp the age of mine would be to check every component for tolerance and leakage and replace anything that's gone too far. He's also going to re-valve the output section for me with Mesa STR440's.
I asked about which components he uses and he said he uses only Mesa genuine parts, which he carries a stock of, but if he didn't have something in particular we'd have to wait for the next container arriving at Westside distribution, as the parts would be added to the shipment.
All in all, I'm looking forward to having my Boogie given the full makeover, I'm taking it to him a week on Saturday.
Dave is in Wakefield BTW.
Cheers,
Rob.