Hi All,
I, too, have made recordings with the SR 71's and 77's side by side. I did find that with the pairs I had, the low-end was a bit more pronounced and the high-end a bit smoother on the 77's.
At Earthworks, as others have indicated, the SR77's were considered cream of the crop 71's. They are the same capsules but the best ones got labelled 77's. Still, it is possible to find 71's that are indistinguishable from 77's.
I hope this helps.
Marc