About 25 years ago, I asked a gentleman who sells NOS tubes (an individual who most on A'Gon seem to have a lot of respect for) about mixing tube brands in a certain application. He told me it wasn't his favorite thing, but not as bad as mixing cables. I asked him what was wrong with that, & he told me (and I am paraphrasing) that if Cable Brand A did one sonic aspect well, but was upstream of Cable Brand B which fell short on that attribute of Cable Brand A, but did a different sonic aspect well, the positive aspect of Cable Brand A was being negated by Cable Brand B's failure to pass it on.
Anyway, that was his logic; he sells tubes so I guess he doesn't have any skin in the cable game.