Going to have a dedicated line installed- What should i know to ask for?

I had a great electrical contractor in Brooklyn when I lived there- sadly, he is in prison for life. When I saw the NY Post article about it (I had moved up to the Lower Hudson Valley by that point), I told my wife, "I’m tempted to go to court as a character witness. And say, ’Look judge, I know the guy was convicted or plead out to some really bad stuff, murder, embezzlement, money laundering, kickbacks, obstruction, etc. but you gotta realize, this guy was an unbelievably good contractor. You know how hard those are to find?"
You’ll be in good hands with @jea48 and some of the others here who really know their stuff.
A dedicated line will not isolate you from electrical gremlins within a system that shares ground with other stuff, even within a private house. It is still a good idea, if only to get a new fresh run, unencumbered by other appliances or connections. That does sound cheap, even by the standards of the guys with, ahem, you know, good connections? :)
good luck,

@french_fries ,
You could do what I did and add a subpanel for your audio lines. The electrician tied into my house service panel and installed two dedicated 20A breakers, Romex, and 2 Hubbell duplexes which are grounded to the subpanel (not self-grounded).

Dedicated lines are not completely isolated, but in my case, the noise floor was lowered and a buzzing sound from an old circuit was eliminated.
And @jea48 helped advise me along the way.

Lots of good posts already so I won't add any redundancy, just will comment that I ran a dedicated (also called a "home run") line to my dedicated listening room. The results were entirely satisfactory. Worth the $, IMO. YMMV

Tom

Sometimes, it's just a matter of luck. Our house was built in 1952, the original wiring was two-wire, w/o ground. The boxes are all metal, and are (from all I've seen) grounded separately from the circuits. It's dead quiet, zero hum.  In the twenty years we've been here, we've replaced the fuse boxes with a modern breaker panel, replaced the original meter with a 400A meter socket, re-did the service drop, replaced much of the old two-wire with modern Romex w/ground, as I've been able during remodels, etc. It was a fairly basic arrangement, done well (for the time), and wasn't ever botched up.My only caution to anyone would be to have the work professionally done, if you aren't certain what you're doing. Goes double, if you're in a multi-family dwelling.

Dedicated line is great. You could isolate the dedicated line completely by having your electrician install a isolation transformer. Square D makes them as well as many other manufacturers. You could even go so far as to place the transformer between your panel and subpanel (if you went wild and installed that too).
But a #12awg (2) wire plus ground, metal clad cable, or EMT raceway with wire to feed a high quality plug will work wonders for the sound quality. I think isolation transformers are the best if you live in a building with many other residents feeding off the same main service you are.