I'm the one who won the eBay auction for the SI-1 integrator. Can't wait for it to arrive!
Prof.: Thiel crossovers can be used with non-Thiel subs. Setup may be somewhat less automatic than with Smartsubs, but no more and often less complicated than using non-Thiel crossovers. For example, the SI-1 makes it easy for non-experts to integrate any main speakers with Thiel Smartsubs, but it has essentially the same controls as the CR-1. Real differences: The SI-1 is fully balanced; the CR-1 is not (only its inputs/outputs are balanced). The SI-1 uses first-order crossovers; the CR-1 uses 4th-order crossovers.
It's also possible to use Thiel's passive PXO5 crossovers with non-Thiel subs. I'm temporarily using the PXO5 that previously fed my Thiel SW-1 to mate my F112s in mono/augment mode with my CS3.7s. The JLA's integrate very well even though I haven't bothered to match levels for this temporary configuration. I did, however, use a rough calculation and the JLA phase controls to phase align the system. This yielded impressive improvements in soundstage and slam and, overall, the best sound ever from my system.
A simple and cost-effective solution for Prof's system, running the subs mono/augment, might be: (1) set up the JLA E110s for master/slave operation; (2) switch off the internal crossover on the master JLA; (3) install a PXO5 configured for your CS2.7s or 3.7s; (3) adjust the level on the master sub to taste (or use measurements). (4) use the polarity and phase controls on the master to phase align your system.
In this setup, the PXO5s handle everything other than level matching as intended. You'd need an adaptor to connect the XLR line-level PXO5 outputs to the RCA line-level inputs on your master sub. You can skip step 4, but you'd be missing a huge improvement.
Note: The 4th-order crossovers in the JLAs are not compatible with the 1st-order crossovers in Thiel speakers. In augment mode, the PXO5's should provide much better integration than anything you can achieve with the JLA internal crossovers.
Check out white papers on the sounddoctor website for tons of expert information on how to set up and integrate subs.
If you're looking for an active crossover, consider Marchand Electronics as a less expensive, but still audiophile grade, alternative to the JLA CR-1. The XM9 and XM44 series are very reasonably priced, but you have to swap plug-in modules to change the crossover frequency. The XM44 offers a fully balanced option that is not available with the CR-1. The XM66 series is functionally very similar to the CR-1 (no module swapping). If you can tolerate the pro audio appearance, you'll be very happy with the cost savings. I was leaning toward purchasing a fully balanced XM44 until the SI-1 came up for sale on eBay.