@mijostyn - I fully agree with your statements on gain structure to get bass outputs at desired levels. An outboard crossover with gain adjustments is a great piece of equipment to help with any type of subwoofer integration.
Back to the original question as to if REL (and other modern powered subs) are musical. Back about 33 years ago, I tried to integrate subwoofers into my two-channel system. I had the outboard crossover and separate power amp setup at my disposal. I brought home at least 4 non powered subwoofers (all used gear that could be returned) from various local stereo shops. I tried everything to gain "musical" low end extension and failed miserably at it. All that was available back then was home theater type subs and all were very tubby and muddy. Great for explosions on the Backdraft movie soundtrack but did not add anything audiophile like for music playback. Even the salesmen back then kinda laughed at me as to what I was trying to accomplish. Fast forward to now and I think these powered cabinets with DSP have come a long way in having the potential to add musical bass to two channel systems without the HT boom.
That said, I think many of these ~$500 - $1,000 subs have compromises due to R&D budget and material cost constraints to meet a price point. "Fix it with DSP" is the remedy. For many enthusiasts, that may O.K. and enjoyable. No harm, no foul. But, to some discerning ears, one can hear these compromises and to @mijostyn 's point, it may not be worth the effort.
Is the money better spent on more full range speakers or increasing the budget for the upper-level offerings from these sub manufacturers...?