I would expect that the 610T would sound more powerful with the KT120s - both the REf110 and the REF220 sound much more dynamic with them. May be the REF610 is already so powerful you do not notice it with the Soundlab's!
I also own PX Soundlabs and the REF610 are my dream amplification system, but heat concern have kept them away from my room.
Can I ask you what frequency you are using in the sub crossover? Do you use it all the time?