I do believe Lindsley and Jackson both had steel string singers. That’s where SRV first heard one I think, at Jackson’s studio. Not sure which s/n those were or which s/n this one was based on but it’s fantastic. I cannot imagine a real Dumble sounding much better.
I bought the pack and found your take on "running on lindley". I took my Tele (actually a replica of Broadcaster #0211) and while that sure is not the Rickenbacker Lap Steel Lindley used on that song, the amp tone nails it. There is a video from that song from 1977 (different from the album version) where Browne plays a Strat with a Tele headstock. And that sound - probably a Dumble as well - I could really nail.
Interestingly enough those are tones which do work in a more contemporary kind of music as well.
Regarding the type of amps I did some research and it seems Lindley has stated in an recent interview that he was using an ODS (actually the second one ever made).
But whatever it was, your Grammatico SSS pack sounds just like it.