I'd try to sell them both and get Mac452 or even dedicated monos.
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I agree with Marakanetz.
It would be best IMHO to buy true monoblocks. Dedicated monos are designed around one input and one output. There is no forcing the amp into doing a job it is really not designed for. (even if capable) I have heard that bridging is far from an ideal circuit and has a proonounced effect on the sound. Do a search of this forum for "Bridging" to get more info.
The SF Ellipsa is 4 ohm nominal, 2.5 ohms minimum and have a phase angle at 70 Hz that eats current. They really don't go that low into the bass and roll off quickly after the port tuning at 32 Hz. So the question becomes, how much power can they use because, unless mismatched to the load, the amps won't change much in character.
With the MC252, you have the mono options of either parallel or bridged for 500W. Parallel would be better for low impedance/high demand loads. Seen special, short wires for this purpose with Macs. Parallel is rather unique for a SS amp. Aside from Mac, I've only seen Bryston offer this and only as a factory option. When bridging amps, you have to be careful about the load because the current doesn't rise along with the power. With a few exceptions, bridging most amps is generally not recommended for 4 ohm loads.
Then again, what could be more macho than a pair of behemoths with big blue lights?