I don't know anything about that particular buffer but I would also try it between the pre and the amp so that all sources get the benefit of tubes and then go from there.
grant fidelity tube buffer experience
Just wondering if anyone has any experience with a Grant Fidelity tube buffer and just looking to see what kind of results you had. Currently I am running a Bryston amp and Martin Logan SL3's. I was thinking of putting the buffer between my Parasound phono stage and my pre amp. Any thought would be great.