I've used GT's rated around "6" in my Mesa's quite often, although my Mesa's are not under warranty, so I'm worried about it. It's true that a rating of 4-6 falls under the set bias in a Mesa. The only issue with ordering tubes (if you want them) is that some places sell them in approximate ratings on-line, such as "GT Rated 4-7." You don't know if they're going to send you a "4," or if they're going to send you a "7." So, if you're going to buy, I would suggest either finding an outlet that will give you the same deal you find on-line for pricing, or find an on-line vendor that lets you pick the exact rating of the tube you want.
As for the warranty and Randall Smith...just call him! 8) I've spoken with Smith a handful of times, as well as Mike B. (and his wife once or twice!). Mesa has great customer service, and all you have to do is give them a call. They'll be happy to help and sort through the issue with you.