Why are you using thick film resistors? Thick film resistors are oxide paste film. They have the most self-noise, worst tolerances, and high temperature coefficients.
Even thin film resistors have better temp coefficients and lower noise. They are made of a sputtered metallic nichrome film and in general sound a lot better.
In a loudspeaker, I would use wirewound with non-inductive Ayrton-Perry windings. The very best would be the Vishay metal film, but those are supremely expensive and difficult to get in the right values.
In some cases, I would use a carbon resistor for a different sonic characteristic.
Try these: http://www.vishay.com/docs/31801/mra.pdf
Using a lot in series/parallel is not recommended. The reason is that you are then subject to a lot of different signal paths depending on the tolerance of each resistor.
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