My KB37 will not transmit a midi signal when using El Capitan. The mic and instrument inputs work fine but the keyboard would not transmit a mid signal into garage band. I tried everything, re-installing software, drivers etc.
I finally found a fix (though not too elegant) by installing Yosemite 10.5 on a new partition on the hard drive (I think this is easier than down grading from El Capitan to Yosemite). The problem with using a different partition is you have to restart to switch between operating systems to use the KB37. Downloaded Pod Farm 2.5.9 from Line 6, installed it in Yosemite, and it works again! What a pain. I don't know if Sierra OS works, my mac is too old to install it. I doubt if El Capitan will ever be fixed.
Note: If you ever downloaded Yosemite from the App store, you can download it again within your purchases tab, otherwise I could not find it anywhere.