Thiel 3.5 upgrade to 3.6 or 2.4 worthwhile?

IMHO, the newer Thiels are smoother higher up, but the older 3.5's have better bass (as long as it's not too loud), are more coherent, and (though high a quality amp with preferably at least 200 Watts into 4 Ohms is needed), even with the eq, easier for an amplifier to drive. I prefer the 3.5's, and think when new and especially used, are amongst the greatest audio values of all time, others will prefer the newer models.

The 3.5 is a high value speaker system. I never owned the 3.5s but listened to it at a dealer's showroom. At the time, it was a great speaker.

I auditioned the 3.6s at the same dealer before I bought it and thought they sounded better than the 3.5s. The 3.6 was more transparent, dynamic, and quicker transient attack. However, in the wrong position, its bass could be overwhelming. It could be too generous and made the images bloated.

I borrowed the 2.4s to audition at home and thought they sounded more neutral. It didn't have the 3.6s deep bass, but its soundstaging was bigger. The images were a bit smaller than the 3.6s. In a smaller room, I would prefer the 2.4s. If you have to position the 3.6s very close to the back wall, the bass would be hard to tame unless you have lots of acoustic treatment.

I won't say it would be a quantum leap in sound quality upgrading to the 3.6s or 2.4s. I personally think the less components in the audio chain the better. So I don't prefer the extra equalizer in the system.

Regardless, you should try to audition either the 3.6s or 2.4s before making the decision. There could be a very nice synergy between your 3.5s and the rest of the components.

I would put the money in a better amp if it were me. Also I find PBJ to be a little sticky at times; sorry the Devil made me do it.