The M50v4 has always been part of the line-up as far back as I can remember.
Prodigious bass! I can use them as my fan in the summer. Very high sensitivity! They can be driven very loud with relatively little power compared to the M5 and even the M3 (the M5 really does need a lot).
For some context, they are in my average-sized living room in 2.0. I am driving them with the Pioneer A9 which is rated at 55W/channel into 8 Ohms, 75W into 4 Ohms. The back and front ports are wide open. BTW, this is not the ancient A9 nor the modern one with the USB port. It's the good one (: The one with dual mono construction complete with twin toroids and 30,000uF per side.
I've been letting them play while doing yard-work. They are shaking my siding on the front of the house and can be heard into the neighbor's yard with the windows closed. Now of course you might ask how much I'm putting into them. The Pioneer is set to -17. At that level, Renee Olstead's Hit the Road Jack is hitting 15V peaks (maybe more but that's the best I can do with my humble meter). It's really hard to say how much power that represents but let's use 8 Ohms as the impedance magnitude. That's less than 30 peak Watts (not a challenge at all for the Pioneer) and I was surprised at how loud that was when I sat down to listen. It was way too frickin' loud so I grabbed the SPL meter to see if it was just my ears being extra sharp today. At 11 feet away, with the tune and level I described above, the M50s are hitting 106dBC!!!! I have to check my notes but if memory serves, the M5s hit well below 100dBC at that level.
Now there's SPL and there's CLEAN SPL. I haven't drawn any SPL vs. cleanliness comparisons yet between the M3, M5 and M50 but I can tell you I hear audible distortion at peaks of 100dBC at 11 feet away with the M50.
There's also the matter of what content might be missing. This one is not hard at all. The M5s are much better in this department just like when I compared them against the M3. Better bass, better mid-range and better highs. Having said that, the M5s are not as easy to listen to in my living room as the M3s and M50s. The main A/V room is a completely different matter. I think the big difference is the hardwood in my living room vs. carpet in the main A/V room.
Then there's the M100. 109dBC at 13 feet away and it's clean!
