Full active subwoofer filter design

Hi everyone,
First of all, I would like to thank the community for the great dissemination done in this forum, I appreciate it for helping me with many aspects involved in the audio processing.
Basically, I am writing to you about my last project regarding the active sub filter. Many years ago, when I was 20 yo, I built a TL subwoofer with the same enclosure and size of the AWCS (BOSE) and the corresponding active filter. Besides my knowledge at that time, I had managed to solve many issues involved in its design (L and R separation,L and R mixing, grounding loops, noise etc). Lately, I decided to improve the mid and high range, thus I need to recalibrate the bass level, consequently the past design is no longer suitable for me, because it was not adjustable.
Looking for this forum I came across in two different posts, as usual I take the interesting part of several schematics, then I modify them in order to fit my needs. In these case, to build an active sub crossover, I decided to consider these two schematics:
https://www.diyaudio.com/community/threads/my-balanced-line-receiver.398440/
https://www.diyaudio.com/community/threads/active-subwoofer-filter.299230/
my design is made up of the following stages:
  1. two balanced line receivers to get the max CMRR and to separate the ground between the external DAC and the main amplifier;
  2. an arithmetic mixed which does the averaging of the both signals (no amplification is needed at this step;
  3. signal amplification/attenuation + - 4.5 dB
  4. second order high pass filter (15 Hz cut-off freq.)
  5. two second order bessel low pass filters in series (the last one can have an adjustable cut-off, both have a cut off freq. of 80 Hz, (the TL sub resonates a lot around 100 Hz thus I need to get a plain response, also a 4th order is the minimum).
  6. Lastly, a phase control in order to accord it with the TL midwoofer I’m designing now, the bessel type of the LP-filter should in theory help to have a plane and effective control of the phase.
Attached to this post there are the corresponding images of the circuit, besides I’ve added a simulation of the final band pass to show the difference between my config and the forum version adjusting the two resistors available in the schematic to get the same cut off freq. in the LP filter (the Sims are carried out in LTSpice). As you can see the Bessel configuration provides a greater attenuation in the proximity of the cut off freq. and a more linearity in the phase variation.

After this preamble, I would like to know if this design is fine or it requires some mods, in few words, what do you think about it? Did I do some mistakes?…
At the end of the project I am going to share the final schematic, gerber etc. using SMD components on EasyEDA platform.
Thanks in advance.

Roberto
 

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