I have two pairs, bought as "matched", but for the differential "work" they probably need some "trimming" (high gm = small change in Ugk=large change in Ia). Fantastic tube, high µ and gm, 1mm thick Ti-anode, Pt- grid wire, very linear, but needs some attention to avoid "transistor sound" (k5) - better run on lower current (by Pete Millett measurements). Then oscillations are higly possible, maybe best to use "stoppers" on both grid connections, and all four cathode connections.
It`s high on my list too (300B driver), but too many other options to try, and I`m too lazy...
Another not so good feature is that it needs bias in the -2...2,5V area, broadly speaking, and cannot "accept" full 2Vrms signal from CD source... And we need about 2*55Vrms input (min.) in 300B pair...not so easy even for 6S45Pi in diff. driver. But, one stage driver sounds good:-).
At least, if nothing concretly, I posted the picture:-)