If you're getting too much feedback playing vinyl, then your turntable is not well isolated. Try putting it on a board mounted on a slightly inflated, small bicycle tube. The turntable will float like a bowl of jelly, placing its resonant frequency below any output from the speakers.Well, I do have an NAD 1155 Pre w/ tone controls and a "Loudness" button and has served me well when I have my tube pre serviced. But now when I think about it, I don't use the button when I play records. It creates too much vibration and feedback even though I have suitable isolation on the stands, TT and room dampening. I would dare say a Sub would create the same issue.
My subs are at least 45 years old, passive designs with both low and high pass filters. I wouldn't be without them. I listen to a lot of organ music, and my friends are impressed on whatever program material they prefer. I myself use M105s which impress them even without a sub.
If your living room or lounge is small, with a length less that 14' or 4.26m a sub will not help extend to the frequency response of your speakers, however, with low and high pass filters, will reduce the FM (frequency modulation) distortion above and allow you to play music much louder.