I have to say I found the mackie big knobs "big knob" very bad. They had one of that when I was demoing some monitors in a store and I told the guy to turn the knob all the way up couse it was so obvious it was taking something away from the sound. I agree though for the price its good (lots of options on it).
I didn't try SPL but would like to couse all comments around here are positive. I'm using my tc electronics passive volume controller and its good (silent and not coloring)-specially for the price.
I'm not sure if this is the same thing thats used in my soundcard but you can also check this out TC Electronic | Level Pilot