Just answering because the topic is live now (the OP is probably long gone)
Mills (brand resistors) MRA series are wirewound non-inductive non-magnetic. The 12W variants are 20ppm (most reasonably priced quality resistors are 100ppm) and 1% tolerance.
Excellent resistors for crossovers, tube amp bias circuits (because they are available in 5W and 12W values).
Mills MRA Series 1R0 12W
Mills MRA Series 3R9 12W + Mills MRA Series 4R7 12W = 8R6 20-ishW (connect in series)*
About $5 each (6 needed for a stereo pair of crossovers)
ppm = the tendency of the resistor to change value with temperature.
" ... Is there a downside to using the 1R0 20w instead of the 1R0 10w ..."
You will increase the power handling of the Crossover, a good thing. Downside? Costs a bit more.
*The formula is simple, N resistors x P watts = NP watts if the resistance values are identical and they are connected in series. However, if the values differ, one resistor will dissipate more power than the other, so 2 of the Mills above x 12W will not be exactly 24W (will not share the power dissipation load equally). That formula is somewhat complex, thus we are using an educated guess here, because I'm in a lazy mood.
Since your original Xor only used 10W resistors, you will be more than OK with making up your desired 8R6 with the two values in series.