General rules of thumb...X pipe will take some of the "bark" out of the sound. Mixes the pules and softens them. Longer muffler with everything else the same will give a lower sound, short will have a higher note. What is an "echo can"? That is new to me. "Resonance" tubes were just a nice name for the pipe that went over the rear axle. They did make a mellow softer sound than dumps at the wheel well. So I guess that is where the name came from. SBC engines have the same pulse dynamic across the line, so any muffler brand that sounds good to you will work no matter if you have a 265,283,327,302,350, or what ever. Glass packs are their own animal. Short ones will get louder and harsher over time. Old school types with real fiberglass will do that faster than the newer types. You can have all the fun you want when comparing different types and manufactures for "ultimate" performance but for sound just listen at some car shows. If you like the sound get the information from the owner. He or She will talk your ear off about this stuff. Enjoy!
