>> Cary SLP-98 tube preamplifier with good 6SN7s << is also a "fairly popular" combination according to Nelson himself in an email I received from him last summer. He indicating having several customers with this combo, fwiw.
A good friend who bought my former tube amp was comparing over 30 days with his buddies loaner Pass XA-25 using his personal Cary Audio SLP-98 (with good vintage 6SN7s, and really good coupling caps) indicating he really liked the synergy with some of the Pass amps. I plan to try it this year too just to test it to see how good it really is.