Speaking of E6 B-units, if you really want A-B-... consist, and you're willing to settle for TMCC over LEGACY, 3rd Rail is taking reservations for their E6 run. Offering both A & B in Santa Fe, both units powered. They're pretty good with road-specific details, too, from what I understand. I only have the GM Train of Tomorrow and a N&W K-2a from them, and a NYC PA/PB pair on order. I love LEGACY, but 3rd Rail is starting to grow on me, too.
Just realized...they're also taking F3 reservations, too.