I dunno man. I A/B different tubes and I certainly can hear a difference. The other way to look at is if I buy 10 tubes and they all sound great, then I've got 10 great tubes. If I buy 10 tubes and 5 sound good, now I'm stuck with the rest that I won't use - point being, I don't think money or justification has anything to do with it.
That's not to say tubes are going to make a crap amp sound great, but they can certainly add that extra something to make your amp sound the best it can. I think if you don't hear a difference between most tubes, you're not listening hard enough or you've lost too much hearing!
As for 6L6s, which you definitely want for a heavy high gain metal tone IMO, Mesa sounds alright. I've heard of people getting better results with SED or TAD brands, but if you want to stick with Mesa out of safety, I believe they're the same as Ruby, which sound pretty good.