I used a Q-filter with a set of L-280s with an L-290 in the bridge. It was a minimally effective control, but it should be continuous in its response - not like an "on/off" switch.

The usual way to have both a master tone and a Q-tone is to make them independent through the use of a push/pull or push/push pot. The switching action brings the inductor in and out of the circuit and allows you control with only one knob. If you want to keep the current layout, then try Pete's suggestion of a no-load tone control. Otherwise, you could swap one of the controls for a switching type and add a mini-toggle to the space formerly occupied by a tone knob. The mini toggle could be used for series/parallel control or something else interesting.

-Ben