Dedicated Line Noise Filter?

For $750 you would be done with the problem, rather than adding another component/layer. Even better if you can buy yourself for less.
Your electrician seems to be a good guy, if he lets you provide materials.
Though, if you let him install the brand he offers, then he will probably stand behind it should anything go wrong in the future. I'd ask him why he thinks his brand is better than the less expensive ones.

The sub-panel is connected by the hot lead and has it’s own ground including a second grounding rod.

Lowrider, my understanding is that a sub-panel should have hot, neutral, and safety ground all wired back to the main service panel, and safety ground (which may or may not be connected from the sub-panel to earth) should only be connected to neutral at the main panel, and NOT at the sub-panel. (Jim/Jea48, correct me if I’m wrong about any of that). Is that how your sub-panel has been wired?

More relevant to your issue, though, have you tried powering the entire system from just one of the dedicated lines coming from the sub-panel? I realize that installing the sub-panel and using its two dedicated lines lowered the noise floor substantially. But using separate dedicated lines for digital and analog components, while certainly beneficial in many cases, in some cases can cause or contribute to ground-loop related noise. And perhaps the improvement the sub-panel installation provided occurred mainly as a result of the new wiring, rather than having separate dedicated lines for digital and analog. Especially if the type of wiring was changed when the sub-panel was installed, as there are several ways in which wiring type can affect noise issues.

Best regards,
-- Al

Lowrider, my understanding is that a sub-panel should have hot, neutral, and safety ground all wired back to the main service panel

Al, that's what I thought. I asked the electrician about code, and he said it meets code and this is how he wired his house for a high-end HT system. He said not having a ground wire back to my circuit box containing old, poor quality wiring would cut down on noise.
I thought, finally I found an electrician who appreciates good audio.
One issue about my main circuit box is that it is a mix of new, updated wiring and old wire and breakers from the previous owners.
Maybe @jea48 can weigh in regarding adding a subpanel.

I'll try using only one line, that's a good idea.
This is an upgrade for me and I wanted to get it right. I used Romex 12/2 with Hubbell 20A brass duplex receptacles grounded (not self-grounded). Romex was run from subpanel taking separate routes to the receptacles, same length.

Get your Self a Industrial Power Conditioner IMHO, Nothing will beat a Elgar 3006.. Atma-Sphere Sells these Pre-conditioned and tested. The ones on Ebay are crap. You should speak to Ralph about this he can advise you on the matter.

I asked the electrician about code, and he said it meets code and this is how he wired his house for a high-end HT system. He said not having a ground wire back to my circuit box containing old, poor quality wiring would cut down on noise.

We'll see what Jim (Jea48) says, as the expert on these matters, but that doesn't sound right to me.  I assume neutral and safety ground are not connected together at the sub-panel, which would be a definite code violation and would create numerous potential safety hazards, and possibly noise problems as well.  Given that assumption, the main purpose of a safety ground is defeated by the approach you've described.  Especially if there is a significant distance between the ground rods of the two panels, and/or the soil in between them is dry.

If a short to chassis were to develop in anything that is plugged into those dedicated lines, that is not "double-insulated" and therefore utilizes safety ground, the path for the resulting fault current would include the soil between the ground rods of the two panels.  The corresponding resistance will likely be high enough to limit the current to too low a value to trip the breaker in either panel, resulting in a shock hazard.

Best regards,
-- Al