Heres the deal: 8 ohm presents twice as much resistance than 4 ohm, so it will take a little more "gain" from a 4 ohm amp to make an 8 ohm speaker sound good. If you had an 8 ohm amp, it would sound okay.
To clarify a bit further, a 4 ohm amp powering a 4 ohm speaker will use about the same amount of electrical power from your car (alternator/battery) as a 8 ohm amp with 8 ohm speaker at same wattage.
The reason you don't want to use 8 ohm speakers with a 4 ohm amp is that for the volume you want, your chips need to run more power through them, which decreases their life, so it'* not recommended.
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Grant
'95 SSEi, few electronic mods (and a stupid coupler)
MiniDisc HU (Sony MDXCA680, European Ver; Sony 4x3ways; 91, 93dB)
Yup, that'* ISU behind Bonnie!
