I have the Ear + HD or something like that. It is maxed out. I have owned it (new) about a year. I love it.
After listening to and enjoying the stock Sovtek as the input tube, I bought some NOS and/or used (who can tell?) tubes made by Raytheon, RCA, Sylvania and GE. The clear winner in this group is the Raytheon triple micah black plates. It had the most natural sound; the most detail; the best bass and imaging. Second place: JHS Sylvania Blackplate SQ getter. Had many of the same traits, but is warm and lush (and i didn't like this in the bass). The RCA and GE, whose actual lineages are very suspect (and so i won't name them) did not sound as good as the Sovtek.
What does the Sovtek excell at? Pace! And it is pretty good at everything else, except for soundstaging. The other tubes cost a lot more, so the Sovtek is a good deal.
I was going to try some NOS 12AX7s, but they are very pricey these days and 5751 prices seem to have stabilised.