[bellicon]

As you likely know, these are among the best rated yet affordable subs.
https://www.rythmikaudio.com/reviews.html  https://www.rythmikaudio.com/technology.html

But some claim to hear coloration which presumably even designer Brian Ding suspects could be due to use of less than ideal op-amps in the plate amps. https://www.rythmikaudio.com/faq.html 

Assuming the plate amp's op-amps are pluggable and not soldered (???), which swappable discrete op-amps might even be better alternatives than this? https://www.youtube.com/watch?v=zyhxKmQy-pw&t=17s

Or this?  https://www.youtube.com/results?search_query=discrete+op+amp

Or which of these? https://www.youtube.com/watch?v=zDA90yMoT18

OTOH, Zockopa commented here https://www.youtube.com/watch?v=cdYFH9ms9EE&t=28s  ,
that upgrading the passive components functioning directly with the op-amp (film caps with dielectrics yielding lower DA and distortion) will likely deliver more cost-effective performance. 

Therefore, are at least the modest, if not brisk sales of this "snake oil" by at least the above three brands driven by placebo satisfaction-rather than vastly less measurable noise?

Otherwise, what else could account for any user claims for perceived larger soundstage, better imaging, more definable midrange and wider flatter frequency response?

And even Brian Ding says here that except for bass heavy dance music, flat response down to 14 Hz (certainly 28Hz) is quite noticeable, as it pertains to recorded event ambiance and the size and proportions of soundstage. https://www.rythmikaudio.com/faq.html#14hz
 
But then the task at hand is how best to upgrade the circuitry of the chosen plate amp?
https://www.rythmikaudio.com/amplifiers.html

[Early B.]

You got questions, we got answers:  https://www.audiocircle.com/index.php?topic=145931.msg1566110#msg1566110

[bellicon]

That seems like a dead thread, along with other concerns.
https://www.audiocircle.com/index.php?action=about;area=accounts

And standalone power amps for these servo subs probably won't work as I think I read somewhere that Brian said that the servo feedback connection has to be as short as possible. If that's true, then Danny Richie, who suggested that the amp could be external to the sub in that thread you posted, should know better, since he markets OB subs with the same drivers and amps.

Too bad Brian Ding doesn't offer these or other optional upgrades to the A370 plate amp-or offer a better performing one.