It happens that as you turn down gain, the treble boost produced by the bright cap is higher and higher. For instance, with the gain pot at noon you have around 20 dB (10x) of relative high frequency boost! The series resistor you added is limiting maximum boost to around 11 dB (3.5x) in this condition.
On the other hand, as you turn up gain, the effect of the bright cap is less and less prominent being nil at max gain. This happens either with or without the resistor you added.
I've been puzzled that no one has mentioned such a simple and elegant solution to the bright switch in amps, particularly when intending to play at low OD levels. Just consider you could put a 3-position center-off switch, where middle position is Bright Off, one side goes straight to the cap for Normal Bright, and the other side of the switch goes to the cap with your limiting resistor in series for a Moderate Bright option.
Glad to hear you are enjoying Thor!
P.D. By the way, I'd like to ask for some feedback on how you are using Thor.
a) Which is your typical Gain pot setting ? (assuming you are using a log or audio pot)
b) Do you use the Bottom switch on or off?
c) Are you using it with a SS or valve amp?
d) Is your amp set clean, near break up, or already distorting?
EDIT: Corrected the maximum boost value with the 220k resistor from 15dB to 11dB.