Thiel 2.7 VS 3.7???

I auditioned both the 2.7 and 3.7 at a local dealer while I was shopping for speakers. Both had a very similar tonal character but I thought the 3.7 was the much better speaker. It was more open and transparent, and sounded less congested on complex musical passages. The 3.7 also had slightly more extended low bass response, but the difference was not huge. Both were auditioned in the same listening room with the same electronics. I know there is a significant price difference between the two speakers, but in this case you get what you pay for.

Let me elaborate on what I meant by the 2.7's sounding "congested". First of all, my comment said it was congested on "complex musical passages". Specifically I'm referring to large scale orchestral music. The resolution of individual instruments in an orchestra is better on the 3.7's. The 3.7's simply handle this kind of material better without sounding strained or congested, probably because its dynamic range performance is superior. But that's what you are paying more for.

I should repeat that I listened to both the 2.7 and 3.7 in the same room with the same electronics, so those variables should be mostly eliminated in comparing the two speakers.

It is possible that there was something wrong with the particular pair of 2.7's I auditioned, but I can only report what my ears heard. I'm guessing that if you really like your 2.7's, then you will absolutely love the 3.7's. I do also agree with one of the comments already made that the 3.7's really need a larger room to "bloom".

If extreme low bass is a high priority, then adding a sub is definitely an option. However, my experience with subs is that it can often be a messy business trying to get them to integrate optimally with the main speakers, depending on the sub you choose. There are potentially many parameters that need to be carefully managed to get it right: crossover frequency, crossover slope, sub placement with respect to room standing waves, sub placement with respect to the main speakers, crossover electrical phasing between the main speakers and sub, etc. I used a sub when I had Thiel CS2.3's, but it took a lot of work to get satisfactory results.

However, if you can manage the price differential, my opinion is that upgrading to CS3.7's will make a bigger overall improvement...not so much in the bass but in most other areas. On the other hand, if you are ready to spend $$, then you really should audition a bunch of other speakers to sample the alternatives.