Best type of Sub to consider?

It seems to me that the answer may depend on where you would want to place the sub(s), which in turn may depend on how much of the bass region you want to augment. And given that all of the woofers in your speakers apparently fire mainly toward the rear, and that essentially half of the area that they are firing toward is non-reflective open space, I suspect that you would want the augmentation to extend relatively high in frequency (e.g., perhaps even upwards of 100 Hz).

If so, I see only two placement possibilities that might make sense, both requiring two subs.

One would be to place a sub alongside the inner surface of each speaker, firing toward the wall just as the woofers in the speakers do. (I presume placing subs alongside the outer surface of the speakers would be unacceptable because it would impede traffic, and perhaps also for aesthetic reasons). Although I would have some concern about the vibrational effects that placement might have on your turntable and electronics.

The other possibility would be to place the subs against the wall, one at its left end and one at its right end, with them firing forward. Ideally the subs would be chosen to provide a means of introducing a variable delay of a few milliseconds into their outputs, so that their outputs could be time aligned with the wall reflections corresponding to the outputs of the woofers in the main speakers.

For either of those placements, and if my suspicion is correct that you would want to augment up to relatively high frequencies within the bass region, it seems to me that two front-firing subs would be the way to go.

Good luck. Regards,
-- Al

One of the brands I am considering is sealed, and the cable it comes with connects directly to my Horning speaker posts. Then, I read on another brand’s website that this is an inferior design flaw. So, another can-of-worms that can be opened up.

In most cases connecting a sub that provides speaker-level inputs to the terminals on your speakers will work fine. The circumstances I can envision in which it would not be a good idea, and might result in hum, noise, or other issues, would be:

1)If the amp(s) driving the speakers have outputs that are balanced, meaning that both the negative and positive output terminals have signals on them.

2) If the amp(s) driving the speakers have outputs that are bridged, meaning that both the negative and positive output terminals have signals on them.

3)In a few cases, such as certain ARC balanced amplifier designs, where the amp’s circuit ground is connected to its 4 ohm output terminal, rather than to the "common" output terminal.

4)In the case of certain older class D amplifier designs, where both the + and - output terminals of the amp are offset from ground by a large DC voltage.

In all other cases, which comprise the majority of amp designs, I don’t think there would be any problem at all doing that.

Regards,
-- Al