if i have a 5 channel amp do i have to use the sub out? or can i leave it blank and use a seperate sub amp for the subs and the other 4 channels for my fronts, mids? or just sell the 5 channel and use a 4 channel.

you don't have to use anything..you can use 3 cahnnels, 2, hell even 1. If you don't need the sub channel, and you can get a good deal for it, sell it. If it's going to cost you, and a new 4 channel is the same as what your 5 channel provides, then don't!

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2004 300M SPECIAL
Alpine CDA 9833
Alpine Pxa-h701
Alpine Rux-c701
Rockford fosgate p400-4
Rockford fosgate p4004
Hifonics BX 1205D
JBL c608 GTi
JBL p693
Infinity 2.5" center channel
Alpine type R 12"(2)
all active SQL set up with 1900 watts rms