I think you answered yourself. You said you already tried a parallel resistor and it helped. Instead of worrying about impedances within the circuit, just listen to the amp. Does it sound compromised in any way? You can tack other resistances in as experiments if you like. That takes just seconds, and your ears will be a better judge then us out here hypothesizing. Seriously.
Same thing with series resistances, you already paralleled the one. Just try whatever else you think might be good. It won't hurt anything, and if it upsets the signal path, you will hear it.
I don't know about your shop, but mine had drawers full of small trimmer pots. But a full size pot works too if you keep wires short. Instead of tacking a bunch of parallel resistors, I just tack in like a 1 meg trimmer. I can them dial it up and down and see what I think. Find a spot you like, pull teh trimmer and measure the resistance you chose.