I would invest in a 500 series rack and look at any of the preamps from http://www.capi-gear.com" onclick="window.open(this.href);return false;. I own and use vintage and modern API preamps(312's,512's,212's) and the CAPI vp26 and vp28 sound spot on. I actually prefer them 80% of the time. They sound huge. They are essentially clones of the preamp used in the 528/536 input modules from vintage API consoles. Hard to beat the price too. These are kits and very easy to put together.
You should also look at the products from Audio Maintenance Limited. They offer a super affordable 500 series Langevin AM16 styled preamp and a 1U Neve 1073 clone called the ez1073. These also requite assembly, but are very straight forward to put together. Best of all they sound amazing and were essentially indistinguishable from original vintage units in the blind testing I have done.