To keep with a more dynamic sound you might try a pair of Paradigm, or Dynaudio if your budget would allow. My experience with Thiel is limited but I agree with some of what you heard. They are a more sweet and pleasant sounding speaker. Some people like that. That's why I said in one of my posts I didn't think they were good for R&R or other high energy music. I think the bass problem has to be room acoustics or placement, although they are a more mellow speaker, they certainly are clear and musical. You will also notice with better gear and speakers that sometimes there really isn't a "sweet spot", the instruments are spread out across the sound stage, a vocal may be off center, etc. I have a Cincinnati Pops Christmas disc that I love but it drives me crazy to listen because at times it sounds like only one speaker is playing until the other horns or something kicks in on the other channel. You will find that a good recording or presentation IS NOT supposed to have everything squeezed into the center of the speakers.

Another thought, did you turn your sub off when listening to the 3.6's? New speakers would require new settings, so turn the sub off if you didn't already. Another suggestion is while you listen to the Thiels take the JBL's out of the room. An active speaker can create like a passive radiator effect on other speakers in the room, especially if you sat them right beside each other.

I suggested Paradigm because they strike me as a lively speaker and they have a higher sensitivity which would make them easier to drive. Dynaudio can be lively when called upon to do so but it takes some juice to make them jump.