You MUST update your Base ROM Object file to 2.90 (this is the most current) if you are updating your K2600 with OS 2.20.  Failure to do so will cause some ROM patches to exhibit a loud oscillating low frequency distortion.  In house testing has revealed that programs 159, 161, and 186 in the K2600 have the most noticeable problems when OS 2.20 is used with any Base ROM Object file before 2.90.  This warning does not appear on the Kurzweil website (kurzweilmusicsystems.com) with the K2600 OS and Object file downloads.