My speakers sound flat with no highs at all

I am trying to work out if you have damaged the speakers and amp too.
Have you tried a pair of headphones on the amp? If they sound ok then your amp is fine.
Assuming it is, then speakers have what's called crossover units in side each of them. This splits the sound into three parts. Bass, midrange, treble. The bass is handled by the woofer's, the midrange by a middle sized speaker or or it's combined with a tweeter, which of course handles the treble.
Connecting a 1.5 volt battery across any of the individual speakers will cause it to pop if working. If the speaker then is not getting sound then the crossover unit is to blame.
When you look at the crossover unit, it will have coils and capacitors (non polarised) on it. The bigger ones deal with the bass and the smaller ones treble. If you find a fault say on the crossover of the speaker, for example no treble comes out of it. Then start by replacing the capacitors. Use the same value as on the capacitor and remember they fit any way round.