Pete
I moved up from the 1.6 to the 2.4. Yes, the 1.6 had a bit more character and at 90 dB was easier to drive. But, I found the lack of bass was its Achilles' heel! The 2.4 has the Thiel signature and should sonically sound very much like the 1.6, perhaps more neutral. The Thiels are extremely fussy with positioning: get it wrong, and they sound dull, nondescript. But, if you get the placement right the Thiels are extremely engaging. Although Jim Thiel himself recommends no toe-in (but is not averse to slight toe-in, depending on room conditions), I have found that toe-in for my 2.4s work well. I toe them in so that from my listening position I cannot see the speaker sides. I am quite intrigued myself why Thiels do not have a bigger following. Most people I know are disdainful and frown on Thiels, describing them as "bright", unexciting, etc. They are absolutely neutral that is for sure and simply expresses sonically what one's system is! One other caveat: adhere to Thiel's recommended listening distance from the speakers. With the 2.4s I find that 9 feet gives me a sweet spot. No nearfiled listening with Thiels!
I feel that Thiels grow with the listener. They don't have that "wow, stand up and boogie" factor but extended listening will make you realise how extremely neutral they really are.
If you want your music to go to the lower, seismic regions and if you have an amplifier to pump enough power, go for the 2.4s. The bass is not bloated or in your face. As one reviewer put it, you feel the bass more than you hear it!
Dennis