McIntosh tube upgrade - worthwhile?

I'm wondering if anyone has ever experienced Sonic improvement when switching from the mediocre JJ stock tubes used by McIntosh?

They tell owners that their tube amps are designed to perform equally well regardless of tubes used.  On the surface this seems incredibly unbelievable.

Anyone notice any changes when they upgrade the tubes? And which tubes do you like to upgrade with. Mostly focused on smaller tubes versus the power tubes which I hear overall don't make much of a difference anyway. But could be wrong.