If you cab acquire the serial number, call McIntosh. They would be the warrantor. Yes Audio Classics may give you a good feel for value. Actually I would eMail McIntosh so you have a documented source should they offer warranty. Chuck Hinton is always very helpful.
The MA6300 is a current model, therefore it is more expensive. I've seen MA6500's between $2,000 and $2,500 in audiogon, ebay and craigslist. A lot of them in like new condition. About the warranty, the amp is too old so it doesn't apply anymore (even if you never filled the form).