A topic dear to my heart, and I only just read this post now.
With my U47 clone (beez neez capsule and components, Telefunken EF12k steel tube), i found myself constantly doing little shoot outs with my pres and decided that the best marriage in the end was the 312 preamp with this U47 style mic. The 312 was clear and really mid crunchy - just like API is tediously described. I hate these silly cliched terms, yet in this case I cant find any better way to describe them.
For the record my pres were:
Baby Animals (jlm 1.4/99v and OEP/99v)
Reliqua 312 (cinemag input xformer, original api 2520, EA output xformer)
EZ1290 - (carnills).
Green Pre (cant remember what else was in them).
Here is what I gleaned from my personal shoot outs - or rather here are my interpretations of them, and these are purely my observations and feelings. They are not founded by the Magna Carta, the Ten Commandments nor validated because Quincy Jones once gave me a hug.
a) The mid range forwardness of the API312 vibe paired very well with the U47's circuit's very pronounced midrange. This truly amazed me and was the opposite of what I had expected. It amazed me because my initial concern, or rather my preconceived idea that two devices which are known to be very mid peaky (u47 and 312) could sound too honky. I feared vocals would sound mid rangy and narrow. In fact it was not the case at all to my ears. They sound so forward and clear. It pleases me so much. Acoustic guitars also sound really crisp and forward as well do vox.
b) The EZ1290 was instantly english 1970's sounding to me, but I felt it was just a bit too intense with the u47 circuit in terms of low mid range. It sounded a little too woofy believe it or not. I actually think the so called "neve haze" is, at least to some extent, about this - in isolation the tracks sound very full and larger than life, but in a mix one needs some serious mixing experience and good ears to get the tracks to sit well without too much mid bass build up. Now I think this was not a problem for top flight engineers back in the day, simply because were highly accomplished and experienced audio techs and knew exactly how to mix good records. I also think they just did it without realising the "haze" since it was simply the desk they were working on. But here's the thing - for the average project studio guy, a bit of EQ cutting can often yield results that are disapointing if not done well. It also adds time into your projects if you feel the mid bass build up is creating a "clouding" issue which your ears don't like. On the other hand, the 312, without the huge low mid thump, seems to sit in the mix the "first time" a lot easier. This for me makes it a winner on its own and I happen to think my ears pretty good at mixing. I know there are a lot of "tried and true" formula ideas out there like API is for drums and bass, Neve is vox and acc guitars etc. I think people who are stuck in these "rules" are often the least talented people when it comes to audio. For me the 312/U47 circuit combo is just lushious. I love it. I am glad I did these personal shoot outs to hear what works for me or what my ears like to hear.
But there is one final comment I make on this -so much is about the microphone you pair with. On my U87ai, I preferred the EZ1290, at least on a single track- it was a larger sound overall. I still maintain though the 1290 would get mid bass build up issues when doing many tracks even with the 87ai, but at least for one off tracks I preferred the EZ1290 with this mic.
c) The BA with the 1.4 xformer sounded much better to my ears than the OEP xformer. I could hear more bass and treble, or at least I thought I could hear that. I liked this pre a lot although the 312 with my 47 clone still seems more crisp and sparkly.
d) The Green pre was kind of boring. Perhaps it should be renamed the "beige" pre
TUBE PRES - I actually think the real macdaddy may be Guavatone's Orange 86. I really do, although I have yet to build it. I have this idea that it would be ultimate microphone preamp combined with a u47 type mic. Now Just like I don't buy into these rules about api for drums and neve for vox (save that for Gearslutz where everybody knows everything), I dont think pairing a tube pre with tube mike is a problem. When I listen to John Lennon's vocals I love the open, forwardness of what I hear, and that is famously a U47 into a REDD47. Its for this reason I suspect the Orange 86 would be tonal nirvana. Just building it eludes me. I should get onto it at some point.
Final point - I actually think any of these preamps can make fantastic recordings. History proves this. Whilst I describe this for fun and because its nice to share my ideas, i take comfort in the fact that I dont take the whole process too seriously and am aware that at the end of the day my ears are what is the most important determinant. Gear is just gear, and I am proud to say I build it just because I love electronics and the soldering iron. The best reason in my mind