rockford fosgate 551x

  i'm not familiar with that exact model, but most rockford amps are not stable into 2 ohms bridged (except mono blocks); they are only stable into two ohm stereo mode.  the old school fosgates are way underrated.  the newer ones may be also, but i wouldn't expect them to be by much, if any.  from what you say, it looks like you will have to run both subs off of two channels of that amp (either 2-ohm stereo, or 4 ohm mono).  you could also try running the subs at 8-ohms each, and bridge two channels up to each sub.  you wouldn't be seeing the full potential of the amp, but the sq would probably be great.  Those subs are def. underrated.  i have put about 2000 watts to each one of the g-5 12's i have, and they love it.  but they sound good even with the gains down, they don't seem too power hungry. (4)g5-1244, (4)gtr12, (2)ppipcx2400, (1)pcx4125, (1)pc4800.2, (3) pair mb quart 6.5"Q, (4)optimas, alumapro (1)5 and(1)15 farad cap, premier deh-p860mp, pioneer xm -dual H.O. alternators