If this indeed due to a voltage increase or decrease, so be it, but it did and does change the sound.I'm sure it does! But that is the only reason they make a difference- and if its all about getting the best sound, you can do better than that by going solid state (and making sure that the circuit and power transformer are happy with the increased current draw) as long as the rectifier/transformer combo doesn't make noise- if it does, there are way of shutting it up.
Every time you think you've surpassed the penultimate, it turns out not to be the case- there's *always* a next step.