Doug verified for me when we were emailing back and forth, that he does indeed mod the line input on the 620, not on the 661. He designed the 661 mod specifically for folks who don't run an outboard pre and run xlr in. Someone asked about PIP, looks like that is intact, but I would get in touch with Doug if that matters.
I have never listened to the stock model, but over the years I've run an m148, two or three warm mod sbm1s, two warm mod 661s, to my ears his warm mods are exactly what I want-warm but detailed. I loved the mod when I ran 4022s, and even more with 4v/41v.