A guitar speaker can be made in 4ohm, 8ohm, an even 16ohm with what I believe is no difference in performance when each is driven by its appropriate power source.
And every change has a pro and con. A higher impedance speaker is easier to drive, but results in less volume at a given power level.
If you mean guitar cabinets, they're wired internally in both series and parallel to obtain the desired impedance, but wiring speakers in series opens a whole other set of sound quality issues, not something that's necessarily a big issue with lo-fi stage gear, but not something desireable in home audio reproduction.
Bren R.
