pulled out my old cheap ratshack meter today and measured the horns. one horn measured 114 dB and two measured 117 dB at the same distance. so to reach nearly double the volume, we'd need at least 8 horns (9 dB increase). 10 dB's equates to double the volume.
i might try hooking up the stock horns as well tomorrow to see how it sounds. two stebels plus both stock horns = really effin loud

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2.4" front/1" rear leveling kit - toyo A/T 285/75/18 tires - bull bar with hella 500FF lights - 20% tint side and back windows - 55% full windshield tint
in the works:
stock HU -> LOC -> PPI FRX-456 -> DLS A4 + Marathon 6150 -> DLS iridium 6.2 comps + Hybrid-Audio L3's + 4 Tang Band W6-1139SI's
to come:
JBL MS-8 + center channel speaker + components for the rear + sirius adapter for stock HU