A bit late, but better late than never. I don't know if the OP already decided what to do (nope, I didn't read all the 13 pages) but here it goes my experience for whose is interested in the matter.
I have no idea how is the copyright stuff around the world but here in Brasil it may be a pain in the ass. I wouldn't make an album cover (or any other commercial artwork) using anything that I wouldn't be allowed to. The reason is because I already was enrolled in a process because a similar thing and it was not fun. Long story short it was a book cover that I was hired to make the artwork with a picture given to me by the client. Happens that she didn't have the rights to use the photo and the widow of the photographer processed her and she found a way to drag me together in a coward attempt of slip off while I was eaten by the wolves. Although I could prove that the client was the real culprit and got inocented at the end I still had a lot of stress and headache, wasted a lot of time in several comes and goes to the court and no one gave me back the money I spent with the lawyer, travelling (it was in another town), etc.
It turned me a bit freak about copyright stuff. Actually I think that copyright is the kind of thing that shouldn't exist. I trully believe that EVERYTHING regarding to culture should be free and humanity patrimon. But there is a huge distance between what I think and how things are. So now I never use something without permission, specially if the owner of the copyrighted thing is from the same country as I live, because if they decide to process you there is no way out. Things may be a bit more cloudy if you are at a different country because copyright laws don't reach people overseas. Well, theoretically they do, but in the real life it's too much more complicated.
A few things to keep in mind in the OP case. First, I think that it depends how the image of the Marshall amp will appear in the album cover. If it is a band picture with the amp somewhat appearing just because it was there in the stage, I think that it may not be a problem. But if the cover is based on the amp, like a front shot it is different. Also, if you are a big kick-ass band that is going to sell millions of copies, maybe they (Marshall) can get upset because you use their product picture and decide to bite some of your profits. Or they may simple love you because you are making a free advertisement as if you were endorsing their equipment. In the case of a small band that will sell a few dozen of CD copies, I doubt that the company would care about to make their juridic department to scratch the butt. Nonetheless it is not a rule. Certain companies like Gibson, for instances, will stalk anyone that they think that may be eating their breadcrumbs. I know of a luthier guy that was building a very limited number of half-scale Les Paul guitars and was told by Gibson lawyers by letters to stop doing that or they would take the 'necessary actions'.
Good luck!
