I have a SP35, and my settings for a clean Mayer tone are as follows:

return fix knob at the back between 11-12, treble 11, Mid 9 (with mid boost on), Bass 12 (deep switch on), Gain 1, Reverb send 12 return 12/1, master 8.30 and countur 9/10.

A caveat: I like a dark tone, and I play at really, really low volumes. For me, the mid and bass boosts work very welll at low volumes as they thicken the sound (I do not need a katana at that point).
If you want to have a thick clean sound have the katana boost on at all times (level around 5/6 - no pull out knob).

The amp is very responsive, so the settings I suggest above can work at very low volumes - but as soon as you increase the master, you will need to re-adjust settings. Remember that there is a pull out bright switch at the back as well (I have not experiment with that thus far).