I'll throw out some additional thoughts regarding the Bags. After having owned the Bags for a few months, it's easier to see what BE's design philosophy was. Let's build a compact, easy to move, and loud cabinet that doesn't try to do everything, but is exceedingly optimized for it does do.
With the Bag S15-D, you have a reasonably light cabinet that is very compact yet contains a 15" driver. The air displaced by the 15" is more than a 10" and less than, yet comparable to, a 2x10. The S15-D is rated at 103 dB efficiency. It doesn't take very much power to get intensely loud with the S15-D. It has tons of usable apparent bass without really needing to pour on the lowest frequencies. In gig situations, it fills out a band and fills up moderate venues.
It doesn't have ultra-lows and ultra-highs, and the lower treble frequencies probably aren't dispersing well because they are coming off a 15". So there are a number of technical reasons that this cabinet shouldn't sound as good as it actually does.
Bag End set out to create the optimum bass cabinet within the design limits they set. And that's what the S15-D is- an optimized cabinet. It's built well, and it is a player's cabinet.
Looking at the S15X-D, which is a bit heavier due to the compression driver and crossover, you've removed the lack of ultra-highs. You have a cabinet that can get very bright and is very present. The only thing that is missing with it are ultra-lows.
We've had a number of threads on TB in 2006 debating whether or not it's imperative to reproduce 40-50 Hz and below at high levels. There are many differing opinions, but a good number of us believe that in live sound reproduction, those lowest frequencies can be reproduced by your stage amp at a level substantially below the harmonic range (80 and up) without compromising the sound. Anyone working with a good PA will have the lowest freqs in the house, without them also needing to be produced on stage.
Personally, I find that both cabinets sound great live.
Nice post. I'll just add that when you stack the s15x-d on top of the s15-d, it REALLY is a nice sounding rig (for all the reasons you mention above), and the coupling, etc. results in a significant increase in the deeper bass. To me, that stack was the 'top of the line' in small, loud, boutique cabs 10 years ago or so. Lot's of other stuff has come along that is lighter, louder, etc., but that stack, which hasn't been updated in many, many years (maybe 15?) still holds its own quite nicely.
