Whenever I use the rate on any synth, I use automation.
You just arm the track (click on it in the sequencer) ie. the subtractor track
then you move the rate knob to how you want it.
This should create an automation lane
In edit mode you can edit the automation and change the rate and how long you want it to last or stay at that rate.
This automation will also be sortof like a block, so you can move it around and have different automation blocks so you dont have to have multiple synths with different rates.
That may sound confusing, but doing a youtube search like ' using automation in reason ' might be useful.
for example this video