A technical question for this community

Whether or not a problem will occur depends on the internal grounding configuration of both the sub and the amp, particularly in terms of the impedance through which their internal circuit grounds and AC safety ground are connected.

By any chance does the M&K sub have a two-prong power plug? If so, you’ll most likely be ok since its circuit ground wouldn’t be connected to AC safety ground.

When a sub and a balanced amp both have three-prong power plugs, as in the case of most modern components, the usual recommendation for a speaker-level connection is to connect the sub’s negative input terminal to a circuit ground point on the amp, if possible, and if not to connect it to a chassis screw on the amp. Connecting to a chassis screw might result in hum, again depending on the internal grounding configurations of the components, but will usually work ok. A circuit ground terminal would be preferable, though, if one is provided on the amp. Or, alternatively, if the amp has an unused RCA input connector a circuit ground connection can be implemented by soldering the negative wire from the sub to the ground sleeve of an RCA plug, leaving the center pin unconnected, and inserting the plug into the RCA connector.

***However,*** your statement about connecting the sub "in and out" leaves me in doubt as to whether any of this would be applicable to what you are trying to do. I assume you mean that the amp would be connected to the sub, and outputs of the sub would be connected to the main speakers. If so, are those sub outputs high-pass filtered, or are they just a directly connected pass-through? Also, I’m not sure what you mean by "this setup currently is not being used in balanced mode." Can you clarify these points further?

Regards,
-- Al

Hi Almarg, the sub has a two prong power plug however the amp uses a three prong plug, maybe I'm ok there as you mentioned, thank you for that.

Yes I would be connecting  the speaker wires coming from the amp to the high level inputs of the sub and then the speaker wires would be connected to the high level ouputs of the sub.

On the other question, let me clarify, although the amp and preamp are a  balanced design, it is not being used in  balanced mode but just using the rca outputs of preamp to the rca inputs of the amp. I am not using the balanced feature of either the preamp or power amp. If this makes a difference.

I don't know if the amp manufacturer is making a warning to the connection of high level (speaker wires) or low level (rca) input connections to a sub, maybe either can pose a problem if the sub has a common ground.

Almarg, maybe the manufacturer is refering to hooking up a device that  grounds its negative wires of the left and right channels to the chassis, would be a problem for a balanced amplifier. I'm just guessing.