Hi. Own the 2.4s. Heard the 3.7s a few times and think they beat the heck out of the 2.4s in all key areas and I love my 2.4s. Can't help you with the comparo to the 3.6s though.
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The 3.7's are a substantial leap forward over the 3.6's and even to the level where the previous top-of-the-line model, the 7.2's were deemed no longer relevant.
I own a pair of 2.4's, and yes, the 3.7's are way better, but also way more expensive and demanding. If you have the room, electronics and of course money, the 3.7's are a real bargin.
I haven't heard the 3.6s but I had both the 3.7s and the 2.4SE and the 3.7s are a big jump forwards. It's actually less harsh that the 2.4SEs and incredible control of the low end. It's also less boomy even in a small room.
But put it in a big room with a couple of SS2s and the PX/S1 to integrate them, and you have a superb full range speaker system.