All good info.
Ground loops (or other ground-related issues that we call ground loops in a vernacular sense) can be a *****, even if everything is done 'right', because even if all the devices are referencing the 'same' ground, the cables connecting them to ground have some degree of impedance, and if devices have current flowing to ground, one way or another, though those impedances, it creates a voltage potential across the grounding cables, and...
Good grounding practices will minimize the paths to the 'same' ground, like star grounding, but sometimes you have to figure things out. I've seen where making adjustments, that in simple theory wouldn't make a damn bit of difference, were like night and day, obnoxious buzz or no identifiable presence of noise beyond the expected low level hiss of electronics background noise.
Sometimes it is a matter of punting with a ground isolation transformer between equipment, sometimes running a ground wire and electrically connecting a chassis to a chassis, providing an additional ground, using a heavy quad shield RG-6 coax for the audio cable, that doesn't change the grounding scheme, but has much more conductor available to minimize the ground impedance than a typical audio cable, separating the audio signal ground from the chassis safety ground (if that's an option provided by the manufacturer), connecting with a balanced connection (which is more likely to transfer artifacts of a grounding issue, if both ends are actually balanced in terms of voltage and input/output impedance), and maybe even creatively doing alternative connections with a balanced connector, playing with floating the ground, or even mixing ground to'-' connections, between Balanced and Balanced or even Balanced and Unbalanced. Audio Control makes Balanced Line Drivers as well as Balanced Line Receivers that run on 24VAC, that you can get on E-Bay used relatively cheap, usually removed from high-dollar custom installations that are getting renovated one more time, that will allow you to balance and unbalance a signal, with gain adjustments on both ends, and screw down terminals that will let you play with lifting the shield as a ground conductor on either side, as well as maximizing signal to noise ratios by adjusting signal levels up to (but hopefully not beyond) signal clipping, and typically have less low frequency distortion than even high quality transformer-based isolators, but if I was going to use on of those, I'd grab a Jensen, but they aren't cheap.
You can try a headphone adapter to RCA to play something from your phone while it's not charging, guaranteeing that there is no ground issue possible, and if that's fine, the sub is fine. In that case, there is most definitely some kind of fix available, even if it might be as esoteric as converting the signal to digital, sending it over an optical cable, and then converting it back to analog.
BTW, I miss my 1259s. Awesome drivers at the price point at that time. one of my favorite things to come out of Ken Kantor, as well as the NHT Super One.
You can also pick up