Pyramid 1800w amp......how many RMS?

My girlfriend recently bought a system for her car, and although i tried to tell her not to, she bought a Pyramid 1800w amp. I'm not exactly sure of the model number, but it's a four channel amp that can be bridged to two channels and will operate at 4 or 2 ohms. The subs she bought to go with the amp are Audiobahn AWT1251 Natural Sound Subs (again, not my first choice, but as she's always fond of telling me "it's not your **** system!"). Anyway, the subs have duel 4ohm voice coils, so I'll be wiring them in a configuration such that the amp is recieving a two ohm load to each channel in two channel mode. My question is how much power RMS is this amp going to produce? The subs are rated at 400w RMS, but I was told by a reliable source that they'll take about 550-600 with no problem. I know that pyramid WAYYYYY overrates their amps, so if any of you know about how many watts that amp will deliver to those subs in the configuration I described, your input would be much appreciated!

Thanks in advance!