YYZsantabarbara - here are some thoughts about Thiel product versions - upgrades follow this general pattern. A product is introduced with components, especially drivers, being prototypes and early small production runs. Within the first year it becomes obvious through 100% production final testing that some tweaking would better center the speakers (made of real, settled production parts) within their window of acceptability. This first revision is generally not announced, since no real circuitry or specifications are changed - merely subtle improvement of more speakers. At some later time, after ongoing feedback from dealers, reviewers, users, staff listening and Jim’s own scrutiny, a second revision is made which is often more significant and serves to upgrade the perceived shortcomings of the product. Sometimes this second revision carries a price increase. In other words the cost / performance plateau is adjusted toward optimum. In some cases the upgrade is significant such as the CS5i (improved); in some cases it is more subtle, but always more optimum, what the product wanted to be when it grew up - the mature iteration of the design.
In the case of the 3.7, here is what I found. In January 2007 the CS3.7 was introduced (original version - I have no layout or schematic.) On Jan 12, 2008 revision 1 was introduced at approximate serial number #517,18. On October 4, 2008, revision 2 was initiated at approximate serial number #881,82. Revision 2 is the current, mature layout.
The changes in revision 2 over revision 1 are significant: 3 changes, all in the tweeter feed section - addition of a bypass resistor, and changes in coil and capacitor values. These changes affect the shape of the roll-in of the lower tweeter to optimize the blend with the midrange roll out, cleaning up frequency and time-domain coarseness in the upper midrange. Note that the 3.7 UMR got some criticism and someone offers an after-market kit to address it (I’m not recommending that kit.)
All in, I would suggest you consider the upgrade if your speakers are under #880 and upper midrange "rightness" is a concern. Note that many rooms with hard surfaces or dimensional problems exacerbate upper midrange glare and ringing. A problem in a ’bright’ room can go away in a ’sweet’ room. Note also that if you're in there making changes: the added 20 ohm tweeter feed resistor and the existing 12 ohm should become Mills MRA-12s, and the feed cap and its bypass should be upgraded to a ClarityCap CSA 16uF at as high a voltage as you can fit and afford ($25-$35 / each). Just my opinion from my hot-rodding experience.
Have fun.
