I don't have the Harbeths but my REX 3 sounds wonderful with my Fyne 704's.
My 704's have upgraded binding posts, Internal wiring and crossover components.
The 704's are really nice stock but these upgrades took them to a new level. DIY SP
I also tried the REX 3 briefly on a pair of Martian Logan Montis.
No issues driving the ESL's as far as I could tell.