I know everyone loves the M2, but I think the M22 is always the better choice; it has the same "sonic signature" and doesn't drop off the map at 70 Hz (in the non-OW model, there's no specs for the OW).
More bass is not always a good thing particularly when a smallish driver is asked to play at a range that taxes it. Also room and placement becomes trickier with extended bass particularly when that bass isn't linear.
Also, the M5 (HP or OW) is rated at 8 ohms, so any standard receiver would drive it; even though it dips lower from 100-500Hz, that's taken into account for the overall rating (so Mojo, you have to explain yourself on that one). I've having no problem driving the M5s and mini-Advents now with a Yamaha RX-V681, and I've used an RX-V673 in the past.
See the graph below. The horizontal line indicates the M5 is more like a 6.3 Ohm load. And it dips below 5 in the mids. Most would get confused by a rating like this so Axiom has chosen to present it as an 8 Ohm load rather than a 6 Ohm or a 4 Ohm. Like I said, it will function with that Onk but it may not perform during dynamic pieces.
https://www.axiomaudio.com/pub/media/catalog/product/m/5/m5hp-impedance.png
