I'd have to say that "best sounding" would be purely subjective. I don't think crossovers make that much difference at idle as far as sound goes. My 67 has a nice little rumpety-rump at idle with a 3" Pypes system with an X. You really hear the difference at higher rpm when the scavenging allegedly starts to happen.FWIW my Howard’s Solid lifter cam instructions stated an exhaust without either will provide the best sounding rough idle.