you could... yes, but don't expect the sub or the speakers to sound their best.
they will make sound though..and you should atleast be using the whole amp anyways
try this.. bridge both sets of channels... now use those two to power your infinitys. start with your gains at minimum and set your highpass filter for 65hz, readjust gains to just below any distortion that you hear at 3/4 vol. if ist not loud enough for you yet (without distortion) adjust the highpass a bit higher then reajust the gain higher. i have the same speakers and they love power.. they rock hard and loud very clean.
I was running mine off of 100w x2 underrated usacoustics (zed) amp. they could have use more! it woke them up and they kicked out the midbass. (i had previously underpowered them because i didn't know what i know now.)
whats your sub rated for? rms wise.. you should probably get a separate amp anyway if you want the sub to do work.
Toreador Red 05' FX4
4.0 4x4 A/Tran Ext.Cab Daily Driver. Nomads, BFG M/T's
Alpine 9857, JBL MS-8 DSP, US Acoustics 2x80w & 2x100w, Vifa 1" XT25 Dual Ring Radiator Tweeters 3k+ 24db slope, Dayton Audio 7" RS180-4 Mid-woofers 100Hz-3k 24db slope, 500w rms Kenwood Class-D Mono - JBL 10" GTO1014 Sealed. 100Hz- 12db slope