Let's take a looksie
Kenwood KAC-X20
180 watts RMS x 2 at 4 ohms (260 watts RMS x 2 at 2 ohms)
520 watts RMS x 1 bridged output at 4 ohms (4-ohm stable in bridged mode)
Specs from Kenwood
2 Ohm@14.4V 260 Watts × 2 (1kHz, .5%THD)
4 Ohm@14.4V 180 Watts × 2 (20-20kHz, .05%THD)
Kenwood KAC-9103D
500 watts RMS x 1 at 4 ohms
(900 watts x 1 at 2 ohms)
1-ohm stable (no power ratings available)
Specs from Kenwood.com
1 Ohm@14.4V Over 900 Watts × 1
2 Ohm@14.4V 900 Watts × 1 (100Hz, 1.1%THD)
4 Ohm@14.4V 500 Watts × 1 (20-200Hz, .5%THD)
Alpine MRP-F550
Specs from Alpine owners manual
RMS Continuous Power (at 14.4 V, 20 - 20kHz)
Per channel into 4 ohms (=1% THD+N, CEA2006).......90 W x 4
Per channel into 2 ohms (=1% THD+N, CEA2006).......125 W x 4
Briged 4 ohms (=1% THD+N, CEA2006).....................250 W x 2
Kenwood KFC-W3011
Specs
Impedance 4 Ohm
Peak Input Power 1200 Watts
Rated Input Power 400 Watts
Max Input Power 1 Watts
My assessment...
You have 2 X 4 Ohm subs.... wire them in parallel, you have 2 Ohms, wire them in series you have 8 Ohms.... I would recommend 2 Ohms... Matching that with an amp.... Subs will handle 400 Watts RMS, I would say use the KAC-9103D, that amp outputs 900 Watts @ 2 Ohms, so not only will it feed your subs fine, it also will be efficient since you won't have to crank it full power to get your subs to sound right, which will keep it from overheating and prolong it's life. That amp is also rated to be 1 Ohm stable too, which means that if in the future you choose to change you're sub setup, your amp should still be able to handle, for example if you were to get 2 x Dual 4 Ohm voice coil subs, they could be wired to 1 - Ohm and you're amp could still handle them... So I'd say go with the KAC-9103D, should be a good investment for the current subs and for future sub choices....
Peace!