Exhaust question???

[Grizzly]

Ok I’ve looked and get anything to come up. I’ve dug around on the interweb and find very little.
2009 ISX @m*m^2’d. Running a straight stack now. Curious if I should go back to factory set up with muffler/resonator which ever it was. Or keep straight or add a spool section.
Which brings me around to spool pipes. WTF are they? What do they do?
I like the look of straight not exactly the sound and I’m thinking I might be giving up proper performance not doing factory (did I jump over a $ to save a .05 way back?). But again with @m*m^2 what works best. Thanks