Hi LL21:
I think he is referring to connectivity of the subs to the high speaker-level signal (amp out/speaker input), which is an option on some subs, like my REL G1’s.
I found that that approach (on the G1’s), muddied up the mids on my Magico speakers - it doesn’t have the LP cutoff that I was seeking.
An alternative is to run the signal from your preamp, as you described, and that is my preference as well, for a couple of reasons: 1) It seems to me that running the input of the sub amp from the output of the main amp, introduced inherent latency, due to an additional amplification stage, and that might be most evident in high impact impulse responses/kick bass/etc. signals, and hence, less impactful attack; and 2) the LP filtering I observed was much more to my liking, reserving the sub’s contribution solely to the lower register (I set the LP to 30HZ Max).
I had another concern with the above approach, and that was the risk of introducing significant negative induced back-EMF (electromotive force) from the large servo/motor/coils, back into the critical low level (XLR / S.E.) preamp outputs, effectively distorting the mains as well. I suspect that is one reason many folks recommend use of the high speaker level connections, as they are a much lower impedance and hence not as severely impacted. I resolved that delima by use of a sub isolator module (an option on my MSB Reference DAC). This electrically isolates the reverse induced sub noise, from the mains by a factor of 1:7. MSB offers a stand alone version, as I recall - I’m not sure if the isolation is the same as on the module.
But the point is, I now can: A) pick off the same signal for my subs, as for my mains; B) minimize group delay/latency; C) alleviate the negative effects of the back EMF; and, D) attain much more desirable LF filtering characteristics, than that observed with the alternative connectivity approach.
Regards,
V2