Hi 1bassleft – Thanks so much for the superb reply. Yea – what you say really makes sense and fits my feeling of unease about using the splitter like that. I was sort of moving in the same direction with my thinking and I was trying to figure out how the amp could “tell” that I’d chaged the impedance to that socket. This evening, I tried sticking the splitter into the ext socket as you suggested and – hey – it worked! …. but only for a while. It sounded great and then when I switched on my Rat – boom! it started roaring – not screaming this time but low deep down roaring. If I keep the volume down it seems to hold it but as soon as I crank it – off it goes again.

Then I also tried putting a “dead” (+ insulated) jack into the ext socket, and putting the splitter into the normal jack like you said and that seems to have solved it. It is definitely louder, clearer and faster – sounds great. By comparison, without the “dead” jack, it’s really muffled and a lot quieter. I’ve turned it up as far as I dare – loud! – and it’s still OK – no screaming, no roaring. Brilliant! 😀 Thanks a lot! I just want to check what you mean by a “dead” jack + insulaled. I’ve just used one end of a lead – should I insulate it in some way?!

I’ve also contacted Marshallamps.com guys. I’ll let you know when I hear back. Thanks again for your detailed and considered reply – much appreciated. You guys sure do provide a great service on this forum.