Hmmm, I'm no fan of matching the Naim 250 with non-Naim preamps, basically because I've never really done it. The 250 is a great amp of course, rhythmic, fast and weighty.
The 606 is a powerhouse and will drive the ESL-63s all day with no problems. However, it's not a particularly fast or rhythmic amp. Therefore my vote would go with the Naim.