LME49990 is not a buffer. It is an opamp. You can design a circuit to use opamp as buffer, but it won't actually be any better than a real buffer. In this case, I see no reason to use LME49990 as buffer, especially since its output capability is only about 1/10 of the stock buffer in E12DIY. That's more like downgrade than anything. There are only two buffers that work in E12DIY besides the stock, the LT1010 and HA-5002.