No idea. I've never played through a 50/50. In fact, I've only had this for a couple days.
As far as tube rack power, I've owned a Marshall 9100, a Marshall EL34 100/100, a 2:90 (for about a week), and this.
I can honestly say this amp is by far the best-built and most rugged of the bunch. The disadvantages are, it's heavy, it's big, and it's not a true dual-mono like the Marshalls.