As far as I know, Brent no longer makes the refurb API 312, and now makes clones and I don't think those use the API 2520 opamp anymore. If you had all of the original good condition 312 parts I think he might still do those kind of refurbs. The only real difference between the 312 and 512 is that the 512 has more updated features and newer version of the 2520 opamp. Some say the older 312 is a tad bit darker sounding than the newer 512 but this could be caused by things other than the newer version opamp. There is not that big of a price difference so I wouldn't bother with 312 and just get the newer 512.

I have to agree that the 312 is darker, but one in good condition has more vibe and richer detail than the 512 to my ear. :p:

Thanks guys. I am going to try one of each. I have 6 spaces to fill in my lunch box. Thanks!