It seems to me that it wouldn't be that hard to "zeel" the ML3. Switch the rca input jacks to BNC ???
you could, but you lose 6db of gain.
in my system the ML3's are alive and feel they have grip and headroom; lower the gain by 6db and it would just be ok in terms of being able to control the speaker. like lowering the speaker efficiency from 97db to 94db (or 92db) plus having to power the whole bass frequencies. with modest power these small things become musically significant.
no doubt the 50 ohm interface has sonic merits objectively all other things being equal. but better it's part of the whole design approach especially for an amplifier. line level gear normally it's fine adding it on as drive/gain is not typically a problem. adding it onto my King Cello was a big plus and transformed it's performance in my system over the conventional interfaces.