Zach - I've had excellent luck with the mufflers from Carter and Gruenenwald. They are MUCH quieter than the Stanleys and not any more restrictive. They also seem to last longer.
If you truly don't care about looks, a muffler from a large luxury car is even quieter. I salvaged one salvaged from a 70's Buick Wildcat and used it on my Super M for a while. It silenced the exhaust so well it was almost eerie. Could now hear all manner of OTHER sounds - valve clatter, the timing gear train, parts spinning in the transmission, etc.