two svs 16-46 or one mirage bps 400?

I wish I could help you. I also have Mirage M5si fronts in my 7.1 HT rig. My subwoofer is a later-generation OM-200, which sits about midway between a BPS 150i and the BPS 400i (it's 2x8 cones driven by a 200w/800w peak amp).

One thing I like about Mirage subs is how musical they are and how well they integrate with speakers, but you're really going for the visceral LFE experience.

My OM200 *almost* gets there. It rattles the room and can make explosions and the like come alive. When I was watching the HD DVD of Polar Express, it sounded like a steam engine was crashing through my family room. I'd expect that going from 8" drivers to 12", and 200w to 400w (the difference between the OM200 and BPS400), you'd start feeling some real chest thump, but I can't say that from direct experience.

By the same token, I've never heard SVS 16-46's.

You might want to read the reviews at audioreview.com. More than one review of the Mirage mentions its ability to make your pantsleg flap.

If you're anything like me, you're probably very used to the Mirage sound. I've had my M5si's for 11 years with no end in sight. The OM200 blended seamlesslessly and was instrumental in turning what was originally a near-full-range 2-channel system into a rip-snorting 7.1 ch. surround which still does music very well.

If I were you, I'd either add a second BPS150i to increase the slam factor or get the BPS400. I think the BPS400 will give you the slam and extension you're looking for while integrating well with your M5si's and remaining musical. If you like music, subwoofers that are only good for HT LFE are a drag. Not that I'm saying that the SVS 16-46 isn't musical--I don't know. But I think you can count on the Mirage subs being musical.