Distortion in speakers- Amp, receiver or speakers?

I've got a 93' 525i and every speaker all of a sudden distorts at low volume. Any good way to tell if it is the amp, receiver or one or all speakers? Two stereo shops claim it is likely the amp, but I sense they are guessing. Any troubleshooting tips?

And if it is the receiver/amp, I'd prefer an exact replacement to keep the factory look- any good sources?

I have a similiar problem and am interested if anyone knows what to replace?
BMW audio is rather expensive so my dad and I would rather replace the right thing from the begining.

thanks.