Subs: DIY Kit vs. Martin Logan

I am in the market for a subwoofer and have been looking to purchase used. My primary interest is in music, it will be added to my 2 channel set-up featuring Magnepan MMG's. To complement the planar speakers I've heard that sealed subs offer the required response. Research has led me to the Martin Logan line-up- specifically the Dynamo and Grotto.

However recently I’ve encountered Rythmik Audio and their DIY servo controlled woofer kits. Rythmic supplies Ascend Acoustic’s subs, whose Sierra product appears highly regarded.

The Rthmik specs for the finished F12:

Driver: 12" high excursion with custom sensing coil
Driver materials: Black or silver anodized aluminum with rubber surround
Cast aluminum basket
Copper flux modulation shorting rings
Amplifier 370w RMS class A/B patented servo controlled
Dimensions 15-3/4"(W) x 15-3/4" (H )x 17"(D) - (18-1/2" D with grille)
Shipping Weight 80 lbs
Frequency Response 14 - 100 Hz (-2 dB @ 14 Hz)
Crossover slope Selectable 12 or 24 dB / octave (PEQ and XLR versions)
Crossover range (Low pass) 40 - 100 Hz
Rumble Filter (High pass ) 14 / 20 / 28 Hz 18 dB / octave
Phase response 90 degrees phase shift at 20 Hz
Phase adjustment 0 - 180 degrees continuously variable
Bass damping factor settings high (Q=0.5), med (Q=0.7), and low (Q=1.1)

Martin Logan’s Grotto Specs:
Frequency Response: 22–150 Hz ±3dB; Anechoic through LFE effects input
Input Impedance: 20,000 ohms
Low Pass Filter Frequencies: 30, 35, 45, 55, 65, 80 Hz
High Pass Filter Frequency: 70Hz
Phase: 0°, 90°, 180°
25Hz Level Control: ±12 dB
Woofer Type: 10" (25.4cm) high excursion, aluminum element woofer with extended throw drive assembly, non-resonance asymmetrical chamber format
Amplifier: 250 watts rms (350 watts peak)
Inputs: RCA line level; RCA LFE; speaker level
Outputs: RCA for multiple subwoofer connection

My question is this- For musically clarity and response, which offers the superior product?
Below are the specs:
I was just considering the exact same two subs! The Rthymik sounds (reads) good, but by DIY woodworking skills are seriously lacking and resale value would greaty suffer.
As you know there are sub cabinet vendors (and cabinet makers).
The better way to go (for me) would be their finished sub as you also mentioned, however they are vinyl not wood, and the signature sub is nicer, but I'm not a fan of the high gloss finish...satin would be better (for me).
I was on the fence about this and leaning toward the finished Rthymik servo sub up until super bowl Sunday when a used Grotto 'i' popped up here on should be here by Friday.The ML subs also have different cabinet shape other then just a square box. If you get back to me after the weekend I can tell you how well it intergrated.
I use to have a ML Decent sub (not 'i' version), but it was too much for my room and not quick enough for some music; the new 'i' versions are said to be quicker and cleaner.
Copper flux modulation shorting rings

This is typically used in speaker drivers that have long coil in short magnetic gap - these type drivers may have high amounts of non-linearity (distortion) due to the voice coil design (most of the voice coil sits outside the magnetic gap or linear part of the magentic field)

Without knowing more specifics about the driver quality on your Martin Logan option it is hard to say.

FWIW, 12" woofer quality can easily be worlds apart from two different manufacturers. Some manufacturers like SVS sounds give you excellent drivers in a low cost box (but cheaply finished and few features) others give you low cost drivers in an expensive beautifully finshed box with rich features.
I've had the Grotto i in my system with Soliloquy 5.0 speakers and a Bryston B60r for about 4 months now, and am extremely happy with it for the price(used). It integrates very well with my speakers, and easily keeps up with them in terms of speed and impact. As previously stated the aesthetics are nice on it as well compared to your standard cube.

Does anyone know if you can order a maple top for the Grotto i? Mine has the black top and I'd be interested in switching it out.

Thanks, for the feedback. I have access to a wood worker in my retired father, so aesthetically I expect the kit option to best the ML. My main concern is minimal distortion and rapid response. Unfortunately, reviews on Rythmik or Ascend Acoustic subs are non-existent.

Shadorne, ML's website offers the following description of the Grotto's driver:

"Grotto's high-resolution driver combines a 10-inch aluminum-cone diaphragm with a high-intensity magnet structure to achieve huge excursion and output capability without sacrificing detail. By eliminating cone flexure, even at long voice-coil excursions, aluminum diaphragms help our advanced-technology bass drivers achieve the very low distortion "

Any thoughts? Has anyone experienced Ascend's subs?
I think ML use a custom 10" Scanspeak - it is a pretty good driver not a cheap one. ML designs gear to work with panels (fast) so it is probably more musical than most subs.
I added the Rythmik F12-SE to compliment a pair of Magnepan 1.6QR's and it is really an amazing sub, never sounds strained, matches well with the maggies and literally seems to not even be there, the hallmark of a great sub. The issue with the maggies is that they are so darn fast that most subwoofers simply can't keep up. Take a look around and you will find that the majority of subwoofers that offer a servo system start in the $2000 and up range which should tell you something. Even the SE version is listed from Rythmik at $899 + shipping and for less that $1000 you are hard pressed to find a servo driven subwoofer of this quality. Also the warranty on Rythmik is something like 5 years on the driver and 2 years on the amplifier. You will love the Rythmik if you own maggies, just my 2 cents.
Which of the above sub woofers has the best bass ? and which has the best output ?