to see if u can hear if the speakers are under powerd, just put 1 speaker in each channel, instead of the 2 that are there now, and see if u hear a difference, if 1 speaker sounds better, then the 2 speakers are bringing down on the quality.
And if the speaker says it is 8 Ohms, then just that speaker is 8 ohms, not the pair, it is 8 ohms PER speaker.
the reason for the 8-16 ohm rating, is because there are 8 ohm and 16 ohm speakers out in the world.
if speakers are put in series, they hafto add up to 16 ohms max for your amp to power them right, (2 speakers hooked together in series gives u 16ohms)
if speakers are put in parallel (only for 16ohm speakers mostly) there impedance is the same, so you take 16, then divided by 2 and u get 8ohms resistance.
so for instance if u put 2 8ohm speakers in parallel, u'll get a total of 4ohms impedance, which is to little to get the full bit of sound, speakers work best at 8ohms, especially if it is a mid range speaker.