Dedicated lines: 1) isolated grounds or not? 2) Subpanel connection wiring

Lem321...my thoughts on your questions...

Isolated grounds or not? - depends on how old your wiring/panel is?
- have your electrician verify how effective the existing ground is
- older instalations may have a grounding issue.
- a licenced electrician will ensure the panel is grounded properly and the isolated ground becomes a moot point

WARNING: "fixes" posted on forums may not be up to code!!!

SUB PANEL: again, I would defer to your electrician, but have him install a 40 amp subpanel. That should suffice for most audio components.

The quality of household Romex today is very good and should not require over sized cable runs between panels.

Outlets: I use Pass and Seymore MRI GRADE 20 amp outlets on a 20 amp line. They clamp like a vice and make a big difference.

NOTE : 20 amp outlets installed on 15 amp lines is not to code and may result in your insurance company denying a related  laim.

I have achieved my best bang for the buck by investing in good outlets and good power cables. But then my supply and houshold wiring is only 30 years old.

Regards

None of it matters, do 240V (balanced) if you can, that will make a big difference.

I do not have a dedicated power line for my audio system but I did install two AC mains duplex: 20A, Cryogenically treated, Rhodium plated contacts wall plugs.  And, yes, much to my surprise, they made a noticable difference. The system is more quiet, has less background noise and I noticed it immediately.  These Rhodium plated contacts wall plugs are recommended ($47 each).  I purchased them from James Romeyn (see link below).

I also purchased his “Pure Grid POWer” RC-8 Outlet Strip/AC Mains Extender ($429). This unit has 4 Cryogenically treated, Rhodium plated contacts wall plugs (same as above) and I use it for my two Hypex NCore NC400 bridged mono block power amplifiers (4 NC400's, 2 each channel) and my DAC.  My Aurender Music Server is plugged direct into the Rhodium plated contacts wall plug. This is electrical power with no surge protection or power conditioning. It is just like plugging my amplifier, DAC and music server direct into the wall plug. His Pure Grid POWer” AC outlet extender is also recommended.  See link below.

http://jamesromeyn.com

James is very helpful and answered my many questions.  I suggest you discuss your dedicated line and wall plug questions with him.