I have many NOS in 12SN7 and 6SN7 and some of the very best but have never liked the grey glass. I find them a little muddy, cover up details and too warm sounding. I would also try something other than the 12AU7.
I had read the same thing about the RCA grey glass, but they somehow are doing about the same or better in terms of detail than the other 6SN7 tubes I have - Sylvania short bottles from 1945 and then some yellow label Sylvanias from I don't know when. Either way, the same exact tubes seem to perform better in terms of detail in my other amps, unless there's some bizarre false perception of additional clarity due to something like noise floor in the other amps - but I don't think so. I also tried some good 12AT7 tubes in the MZ2, and they didn't really add anything in terms of clarity - just a bit of gain, but performance didn't change much. The La Radiotechnique 12AU7 tubes are crystal clear, which is why I like them, and it's my preferred input tube on the ZMF Pendant. I have a pair of mid-tier Tung Sols coming soon, and I'm on a wait list for a pair of Tung Sol Black Glass Round Plates, but they're impossible to find as a true NOS matched pair. What are your favorite 6SN7 tubes?
I'd love to try out the MZ3, regardless, but I haven't seen one on the used market since I started tracking it a month or so ago. I'd be willing to buy new if I were positive I'd love it, but just not sure.