I have five of the X12 500 Series preamps. When I bought them, they were still offered as kits. Once he joined the VPR Alliance, he no longer offered them as kits. They were nice kits to build, seem high quality and they work as described. I don't regret the purchases, however I did build them before I knew about CAPI. I will eventually replace them with CAPI 28s so as to match the CAPI EQs & Comps in my sidecar. Therefore, if anyone might be interested in buying my X12s, let me know.
I too built a x12 and its a hiqh quality 312 esque mic pre. I built the rack version but its nearly identical. I bet one of those sk1's with an op transformer would be a damn good preamp. And for the price? Fugehdaboutit.
I just built the NK-72DI kit, with the separate AML order for the two Carnhill transformers. I chose no options. Came to a total around $250. I marked the front panel with console tape and a sharpie. Right now it's set up with a GAR918BC Jensen-style op amp and it sounds laid back and 3-D huge at the same time. The 2520 op amp was kind of boring to me in this circuit. It's pretty exceptional on bass DI and acoustic guitar so far. Not so much lead vocal, too relaxed.
Overall, I can recommend the FiveFish kits generally. They seem well designed and are pretty easy to build. The owner of the company also seems very passionate and bright. I'll probably do another NK-72DI. I wish he would simplify the code names of his products, they are confusing and hard to remember.
Based on my experience with the SCA T15, which is another THAT IC based preamp, I think my smartest move will be to avoid these kinds of pres altogether in favor of 'slightly' more expensive options. For example the AML ez AM16 Langevin-style preamp clocks in right around $200 at the moment. CAPI VP312 also. SCA C84 also. I think the C84 is going to be my "clean" preamp, in lieu of the T15 or any other version of that circuit. If you're going to go transparent, why not go all the way.
My logic is, why waste valuable rack slots on middling IC pres when I already own an 8 channel Yamaha MLA-8, two Apollos, a Tascam UH-7000, a Presonus DP88, and so on. I'd rather buy another UH-7000 than two THAT pres. I'd rather buy another DP88 than three of them.
Maybe if you don't have any clean preamps this could be a smart option, with a nice DI. I just don't think it's going to win any blind shootouts over the other ones I mentioned.