Amazon.com: Gravity 4X WZL8 8″ Midrange Bullet Loud Speaker 1000 Watts Peak Power Handling 4 Ohm Impedance 106 dB Sensitivity Car Audio Speaker Attached High Energy NdFeB Magnet : Electronics
Design Requirements for Amps and Speakers, Part Three: Low vs. High Impedance – HARMAN Professional Solutions Insights
Impedance Matching of Audio Components
Amazon.com: Gravity WZL6 6.5-Inch Car Audio Coaxial Speaker Midrange Bullet Loud Speaker 800Watts Peak Power 4 ohm Impedance Per Voice Coil with High Energy Magnet Composition : Electronics
Amazon.com: PyleUsa Two-Way Component Speaker System - One Pair 6.5'' Two-Way Component Kit, 360 Watts w/ 4 Ohm Impedance and 1'' High-Temperature ASV Voice Coil (Pair) - PL650CBL : Everything Else
Understanding Speaker Impedance - Geoff the Grey Geek