I'm guessing most of the diff you are hearing between the D1 and this is due to the output stage. I wouldn't personally bother with opamps (other than maybe an OPA861 transconductance stage), but would go for a discrete stage. Zapfilter would be the easiest, though probably not the best. If you do decide to go the Burson route, I would probably do one of the Audio Gd ones, as I suspect that they actually make the Bursons, and theirs are more reasonably priced. Note that for I/V duty, a discrete opamp << a discrete zero feedback I/V stage.