Delta told me that rotary speed controls are simply not compatible with their fans. They did not elaborate but I assume it's because the motor is a DC. I have another bathroom fan, Panasonic I think (will have to look), and it runs perfectly fine on a speed control. So my options are maybe see if I can change out the guts of the Delta fan and try and get a Panasonic fan assembly in there (probably a very bad idea for code reasons). Second is tearing out the complete fans, but this is a lot of work, but do-able. Third is to install an inline fan in the attic space. The attic space is tight and awkward, but do-able and possibly easier than a re&re of the existing fans.
Solar & Palau said that you can use a rotary speed dimmer on their inline fans. They said that noise is a common with a speed control and there is nothing that can be done. Interestingly, the rotary speed control they sell (#SC-15) is a KB Electronics that I installed, but is re-branded for S&P. The options here are to live with the noise (no) or install a silencer in the downstream duct after the fan. I thought Fantech would be an option as they state right on their website that their fans are "100% speed controllable" but I phoned them and they also said noise from the controller is typical. A manufactured silencer is expensive but they are simple to make so that's what's going to happen.
Thanks!