I see Fitero has already made the point I was going to make. You can't beat the CV181s on the input side, but they are VERY expensive, and they do have some QC problems.
You might get great ones, or not. I haven't heard from anybody that has them that doesn't think they sound amazing.
About 90% of them are good, so if you want to take a chance, better pay up for a spare or two. And I agree with Kentley, you don't need them
and likely get better sound with something different on the output buffers. For my limited budget, like it or not, I still like old Sylvania chrome domes. Some think they are bright,
but if you might think they are, mix them with something else on the output end. I can buy a dozen used chrome domes for the price of one CV181.
Preamp tubes are gonna last 5-10 years anyway, so I don't care if the used ones only test at 90% of new.