Well I will start by assuming this is a single speaker unit?
The ohm unit refers to impedence not resistence only, this means that when the coild is energized the coild will generate an inductive impedence along with the resistence of the length of wire.
I would look to match the speaker's impedence rating as the amplifier is internally matched, so if the original speaker is rated a 4 ohm, then replace it with the same. If you use a 2 ohm speaker the amplifier will likely run hot and burn out. The larger impedence of the 8 ohm speaker means that the amplifier is having to use twice the output to obtain the same volume as before, so there may be a greater harmonic distortion noticed by this.
Oh an one more thing, did you make sure that the speaker was in phase?
I hope this helps, if not please let me know and I will try to dig through my memory for an answer.
Last edited by Lang75; 06-07-2012 at 04:00 PM.