I own the 1.6's and am extremely happy with them. Their midrange and highs are excellent and very detailed. The bass is tight and accurate, but does not extend into the low end. I do have a subwoofer integrated into my system and have been able to match it very well with the 1.6 to get that extended bass response.
That being said, the 2.4's are superb. The 3.6's are also excellent speakers that have been a staple in Thiel's lineup for many, many, years and are just now being replaced by the 3.7's which are not officially released yet.
Thiel speakers require good quality amplification to sound their best. I originally drove my 1.6's with a Denon AVR-3300 receiver which surprisingly worked quite well. The downside was that the amp in the receiver tended to get hot and the sound was a bit thin, but it did work. I ended up buying a used Acurus 100x3 amplifier and that has made all the difference in the world with these speakers.
As to why the 3.6's have dropped into the same range as the 1.6's on the used market....I think it's because the 3.6 is now discontinued due to the imminent arrival of the 3.7, and also that the 3.6 uses older driver technology as compared to the rest of the line. The 1.6 is a current product still.
