Well, just a quick look on paper at the B&W you said you heard and liked:
3-way, 200W power handling, 89dB sensitive, 8-ohm load (more like 4-ohm with a pair of narrow frequency peaks at what looks like the crossover points if the graph I found was accurate
https://audio.com.pl/images/9/2/0/20920-max_bw_cm_lab.jpg), aluminum dome tweeter, kevlar 6" midrange, 2x6.5" kevlar woofers.
The X-MTM you mention are a 2-way, about 90Db sensitive, 8-ohm impedance (and knowing something about Danny's designs in general, this is probably a more honest 8-ohms and a pretty flat curve, meaning these should be easier to drive if you don't have some hefty amps feeding them), and use a fabric dome and a pair of treated paper 6.5" woofers.
Both speakers seem to get down around where you want to be in terms of low end extension, but I'd think they are going to sound noticeably different from each other. I'd agree that the X-MTM are probably the closest to an apples-to-apples kit by comparison (in terms of sensitivity and the amount of air moving on the low end), but considering the above saying they are similar by comparison is really a bit of a stretch.
I think I'd agree with others here that for what you're after, a pair of these (or the monitor X-CS cousins) and a pair of subs would be the way to go, but I guess a better question to ask would be how loud is loud? I can't doubt the X-series speakers would be at a disadvantage as far as sound quality (albeit different), but I have to wonder if they would run out of gas in terms of sound quantity before the B&W assuming the B&W have a big/heavy enough amp behind them.
Two alternatives from Danny worth mentioning, I think
If you've heard or could get behind the idea of horns, Tyson just had Danny put together an upgrade kit for the current model Klipsch Forte III, which I think would be right up your wheelhouse in terms of maximizing a setup for the music you like.
Also, I did a quick search on the B&W C9 and found the current comparable model is the 703 which seem to retail (new MSRP) at about 4K a pair. I don't know exactly what your budget is, but there are a couple of really solid deals on folks selling used LS6 right now (the "little brother" to the LS9, although calling the LS6 "little" is again a stretch), and if you can swing around $3K to pick up a pair of those (and can arrange your room to sit far enough back from them) I think you would end up with a speaker for your music tastes that you would have to spend A LOT more money to try to better, and even then there is upgrade room to build up the stock crossovers and internals of a stock set of LS6 to get even more out of them before you'd be at considering a whole-speaker upgrade. Even further, again, specifically thinking about your music tastes (I did an extended demo years ago with a selection of similar music on arguably the most decked-out LS9 ever built with a stupid amount of Raven audio tube amplification to drive them), I'm at a loss off the top of my head as to what you might change these out for that could decidedly better them on the market today; at this level of speaker, a lot of times you're looking at lateral moves in terms of the little details than decidedly "better" speaker upgrades.
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