No idea what may have caused it, but the Marantz range is very reliable; I've had a lot of their gear over the years and never had a problem with any of it. Things go wrong with electronics from time to time, so it'd be a pity to throw the baby out with the bathwater if you generally like the sound of their gear.
If you like the 7200, check out the repair quote and decide from their. If it costs £100 to have it fixed, then that's the cheapest solution of the lot, no? You get your amp back and working again, you have the sound you like and you don't need to spend at least a similar amount (and probably a bit more) to better it.
EDIT: Incidentally, do you mean the PM7200? There's an SR7002 which is AV, but I'm assuming you are referring to this amp below (albeit this is the KI-Signature version):-
http://www.superfi.co.uk/images/PM7200_KI_550.jpg