just a quick glance, but it looks like you ran it series-parallel. i'm assuming the subs are dual 4 ohm subs (those are usually the ones available on the market), so in the end, the amp will see an impedance of 4 ohms. if that's what you're after, then you're good to go.
if the amp is stable at 2 or even 1 ohm and you want a louder setup, then you can run the subs parallel, get it down to 1 ohm...or get different subs - either 2 single 4 ohm or 2 dual 2 ohm subs. whether you use all the speaker terminals on the amplifer...i dont think it matters because (correct me if i'm wrong), most of the 2 channel amplifers are internally bridged. they just give you the option to run stereo or mono....i may be wrong - i've been out of the industry for some time now.. let us know how it sounds!!