Dedicated Line Noise Filter?


I don’t know if what I’m seeking exists, so I’m asking the experts.
I have 2 dedicated lines installed; one for analogue, one for digital. They originate at a sub-panel off the main circuit box. The sub-panel is connected by the hot lead and has it’s own ground including a second grounding rod. There is a home surge protector installed at the main circuit box.

I have passive conditioning on both lines for the components, yet I’m still hearing noise thru the speakers (from about two feet away). I live in a city and I suppose it could be dirty power from the grid.
Now to my question; is there a noise filter that can be installed at the site of the circuit panels? My electrician offered a surge protector with noise filtering (plus EMI/RFI), but was very expensive. So, is there a device that can be installed upstream to lower the noise floor?
Many thanks.
128x128lowrider57
Oh, and @jea48 , I have a 2 phase panel with 60A service (small row home and still meets code). Electrician installed a 60A Double-pole breaker in the main panel, then ran wire in a conduit to the subpanel.
It's about 5" to the side of the main.
Not that it will help, but gotta say, @jea48 is just so generous with his knowledge- he helped me with some questions I had about an isolation transformer install, and I had a big electrical contractor do the work, open a permit, etc. There is a gap in knowledge, I think, between the electrician community and audio-- and jea48 gets "it"- no code shortcuts, but getting to the issues.
I’ll watch this thread with interest. I always learn something on these electrical/code/wiring threads.
What’s scary in Texas (unlike NY) is that it is caveat emptor for buyers. When we were looking at houses here, I’d say, hmmm, I wonder what the file says, and my broker would pull it, go to the city or county as applicable. Outbuidlings that were never permitted, additions, ditto. So, when you buy, it’s on you, the buyer. You may be ok until you want some work done, and then a city official comes by and says, "sir, that outbuilding there- we have no record of it." And you are in backwards compliance land.
I lived in PA many years ago, but have zero knowledge of practices there.
Good luck with this low-- you’ll get it sorted.
bill hart
Many thanks, @whart . I agree that @jea48 has the knowledge that crosses that boundary between electrician and electronics. All that and I bet he can cook. 
Question for @jea48 or @almarg ...
The sub panel feeder Hot/s, neutral, and safety equipment ground conductor are all fed from the same main panel.

The sub panel feeder neutral conductor/neutral bar can not be bonded, connected to the sub panel metal enclosure or the equipment ground bar. The feeder neutral bar must be left isolated/insulated from the metal sub panel enclosure. (Green bonding screw or neutral bonding strap that came with the panel discarded.)
If the the feeder neutral conductor/neutral bar is bonded to the enclosure/equipment grounding conductor/equipment ground bar then the equipment grounding conductor will be connected in parallel with the feeder neutral conductor. What happens then is the equipment grounding conductor will carry neutral current back to the main electrical panel, or where the service neutral is bonded, connected to the enclose, equipment ground, and earth.
Not good for the equipment grounding conductor to carry neutral current.

If the subpanel (containing breakers for 2 lines) takes a feed of hot, neutral, and safety ground from the main panel, are these 2 new lines truly dedicated?
  IOW, by tying in to the main panel (which may have shared neutrals and grounds), is there a chance of causing a ground-loop in the new lines originating from the subpanel?