Hanwell is good and would has a similar form-factor to the word "Marshall', so I'd go with that. "Cleartone" out of CMI would also work.. I don't think that'd be in breach of anything.

I'm no lawyer but isn't there a fair use and hobbyists thing....? I was under the impression that if you're using the likeness of a significant brand in some form of fan-tribute, ie for no commercial gain whatsoever, you should be all right. Also, if in the unlikely event they found out, your intentions are made clear in this thread...you're doing this because you respect the brand, not because you're trying to make money off it! And now that I think about it, they probably have a limited edition Metallica in the Marshall office and if they do see this, they might think it's awesome and order one :pp

But I really don't know what I'm talking about here..