FWIW, my vote goes to OB/Dipole subs to mate with Magggies for best music integration.
I have tried various box subs over the years (dual Martin Logan Servo Descent I's, M&K's, Rels, Magellan/TBI transmission line subs, etc.), but they never blended seamlessly for me so they were either repurposed for HT, returned, sold or turned into dust collectors. They would always show themselves when you try different volume combinations or content selections. What setting/configuration would blend well in one scenario would not in another. The TBI's were nice at low volumes, but would not keep up with the Maggies when you turn up the volume. Port chuffing became the wart at increased volumes and they all gave localization queues which drew attention to themselves. I understand that TBI has made mods since to address the port chuffing, but haven't tried their newer models.
If you want something to blend well with OB/Dipoles, I assume you have to extend similar design characteristics throughout the frequency range, else the relative levels will not match as SPL changes. It is also beneficial to match line sources with line sources and point sources with point sources for similar reasons based on SPL reduction over distance.
I have been playing with 2 - 12" GR Research Dipole Servo subs both with the Rythmik Servo Amp (hardware low pass dipole roll off shelf and servo) and an ICEPower ASP1000 amp (software low pass dipole roll off shelf with no servo). Both have given the best integration to date with my Magnepan 20.1's. I am using a Marchand XM44 XO to cross them over with symmetric 40Hz/48dB slopes. I am getting relatively flat in-room seamless responses down to 13 Hz. After testing these in different OB cabinets and getting the results I have, I have placed another driver on order to make the double into a triple and will possibly add another triple (for stereo) when time and budget permits.
They seem to track with volume changes as well as content changes and do not provide localizations in my venue like the other box subs that I have tried. They have added tangible bass at low volume levels which is something I have never been able to do with Maggies without cranking the volume. In short, I am very happy with their integration (best I have heard) to date and don't feel I will be turning them off because of content specific or SPL level miss-matches for music.
FWIW, I am currently running the 2 drivers in a triple 1.5" thick (sides, center, shelves) MDF H-Frame cabinet that weighs a ton and has no flex. All joints have dados.