I've been reading posts about the benefits of using transformers on the inputs of the Calreq Eq and the SSL Comp, and I know they are a crucial part of the Neve et al sound.
But I was wondering about the use of transformers on the inputs of the mic pre's that I've built, namely the Kev's 2017based units and the Peter Cornell Green Pre's.
Would these benefit from some iron???
If so what would I use (I read of using 600/600 on the SSL).?
If you're thinking of bying iron for it, then I think you're better off doing something new from scratch. The circuits you talk about are already doing away with the transformer; as in they use caps to block the phantom, and they already have a differential circuit that will give you low noise with a typical mic impedance. In transformer mic input circuits, the transformer is normally used to block phantom and to provide excellent common mode rejection, as well as to provide suitable impedance conversion to whatever active device follows it. If you're willing to pay for the iron, I think you may as well go all the way and do a separate amplifier for it. It will give you another colour to use in addition to the ones you already have.
Good luck with whatever you decide!