Sand cast is the lowest grade resistor. Metal oxide is better, but still not as good as high quality non inductive wirewound like mills or mundorf. Graphite is the best.
Shunt is technically not in the signal path by semantics, but it still affects the SQ. Even in shunt Mills will be audible. It's OK to skimp on midrange resistors, but don't skimp on the tweeter resistors.
Silver plated copper is fine. Insulation is more important. Teflon and polyethylene good, PVC bad.