500k is standard for buckers, 250k will roll of some treble, 1meg give you more treble
I'd try 500k, if you want a touch warmer sound, drop to 250k.
When I'm going for a Jazz tone I usually roll the tone off just a touch and drop the volume a bit, it darkens it off just enough - something I picked up for a rather talented gent called Adrian Ingram.... And keep the amp treble up, watching how much bass your using also helps to retain definition. Also, with Jazz, what sounds good at home, ie tones of nice warm bottom, really pisses off bass players in a trio etc...!!
My Stormy sounds great, but I'm going to have to get some Mules by the sound of it....