Check with the tub manufacturer for possible fancier controls...some can be adjusted. Did you check their website to see if that was offered as an option?
I called them actually and the 'Adjustable' pump is an extra option you can opt for when buying the tub. I bought mine off of craigslist so no options for me. They said to upgrade mine at this point would be $500.. I don't need it THAT bad..
The guy I bought it from thought maybe putting a valve in-line with the exhaust from the pump would work but I wasn't sure if that would be hard on the motor or not.