So my dilema is do I upgrade the 2.4's to 2.4SE's as I can get the
upgrade kit from Rob and be content with my current system or do I sell
the 2.4's and the SS2( hopefully to someone who loves Thiel and
appreciates the quality) and get a pair of 3.7's?
The sonic part of the SE "kit" simply replaces the 13 uF polypropylene + 1 uF styrene bypass with a 14 uF Clarity SA and the 27 uF PP + 1 uF bypass with a 28 uF SA. Those are the coax feed caps and SA's were chosen by Jim Thiel and Gary Dayton as the best sounding circa 2008.
But there are better caps available now (eg Clarity CSA) and other passive parts (caps in other positions as well as resistors and coils) can also be upgraded. That is what Tom Thiel is working on. If all goes well, this "super" upgrade kit will be available from Rob Gillum by the end of the year. If you decide to keep the 2.4s I advise waiting for the more robust upgrade kit. "Just" upgrading the resistors made a very nice improvement on my SEs and I suspect the full upgrade will bring the SQ to the next tier.
That said, if you have the coin I recommend the CS3.7 which has better drivers and cabinet than the 2.4. And Tom Thiel might have an upgrade kit for those boards, too, although the time line is further out. I suspect a tricked out CS3.7 would be sonically competitive with some of the very best speakers out there.