well since completing my ZSP1 I've listened to a quad of used 6dj8's that I picked up a while back, they sound really good but I had to shuffle them around a bit because 2 of them were a bit weaker then the other 2, ultimately I found spots for them where they worked well. I have a quad of 6n1p's, those sound a bit brighter to me but otherwise sound very good too. Well I found that the 6n23p is supposed to be identical to the 6dj8's and so should work perfectly. Well I got 10 of them for about $2 each incl shipping so I gave them a try.

They sound excellent, they need some hours on them but they are at least as nice sounding as the 6dj8's but I don't have the problem with any weak ones. Gonna let these ones get some hours on them and we'll see how they sound, but if what I'm hearing right now is any indication these are only going to sound better.

Just a little update, countless hours on the ZSP1 now, I've had a chance to compare it to several other preamps, one that I'd built previously based on a 5687 which I previously thought sounded really good... the ZSP1 is miles ahead of anything else I've heard, dead silent, wonderful detail, very natural sounding, it has taken up permanent residence in my vinyl rig and even my wife has commented on how almost magical that system sounds with the ZSP1 in there. Still running the 6n23p's, nothing sounds quite as correct and natural than these to me at least.