Combining, or "sharing" the Neutral Leg (white wire) can cause an imbalance of a voltage on the Neutral Leg. When a homeowner, or their "handy man" add an outlet, or a light fixture, or another modification to the home wiring, it is tempting to attach the modification to an existing circuit. It is not unusual to find a junction box with multiple neutral wires all connected together. The Neutral Leg has a relationship to Ground, but it is not to be treated like a ground. Stray voltage on the neutral can damage sensitive electronics, especially equipment, like computers, and electronics that operate on a reduced voltage (a power supply transformer is an indicator, which includes wall warts). Voltage on a neutral can indicate a problem out side of the distribution panel, yes. But more often it indicates wiring errors within the home. Stray voltage on the neutral is a problem, but usually it is a symptom of a problem with the basic wiring not being NEC (National Electrical Code, or NFPA 70) "Code" compliant. A separate distribution panel for your listening room does have some advantages, but it needs research and design to be of a true benefit. A qualified Master Electrician can, and should, inspect your wiring and panel for defects and deficiencies before proceeding. In older homes the wiring and protection (circuit breakers age, for example, and wiring and devices may be out of date and improper) may not be in compliance with current NEC "Code" requirements. It is worthwhile having a Licensed Master Electrician (there is stringent and frequent testing involved in moving from Apprentice, through Journeyman to Master. You don't get to self assign that title and award yourself a license) inspect the electrical service in your home and review with you a course of action, if needed. For less than the cost of a "power conditioner" you can have some certainty about the true condition about the quality of the electricity in your home. And then you can make an informed decision on how to proceed.
Measurements for a dedicated line
The question of whether a homeowner should get a dedicated line is often like "should I get bangs." It’s a little complicated. Here are a couple of reasons to consider not:
I. My experience is that you won’t eliminate all the other noise coming from your home even if you do run a dedicated line. I still hear motors switching on and off despite being on completely different circuits.
II. A little resistance and a little inductance may actually be a good thing in keeping noise out of your line, so overkill on the wire gauge may not help this.
Why you definitely should get a dedicated line, with thicker wiring:
Less voltage sag.
Voltage sag means that under load the resistance in the line will cause the AC cabling int he wall itself to consume some of the AC voltage, giving your gear less volts to work with. This sag is proportional to current, so the more amps your gear is drawing the more sag.
This sag is something you can measure. There are two things you need to look: The hot to neutral voltage and the neutral to ground.
With nothing on the circuit your N-E (neutral to earth or ground) should be 2V or less. If it’s significantly higher than that stop and call an electrician. That’s true for any circuit in your home. High N-E values are indicators of a problem which may be in the circuit or in the service wiring from outside to the panel.
What happens when you turn your equipment on and play music is that the line will sag. The H-N (hot to neutral) voltage will drop, and the N-E will go up. Some sag as you turn on big amps is normal. So long as you are not tripping breakers you are fine. What you want to measure is the sag after your system has stabilized and while it’s playing music.
Keep an eye on the N-E value, as this will be a good indicator of the sag independent of the incoming line voltage. It may also point out where you may have issues. That is, if you measure an extra 2V of N-E, your sag is probably around 4V, so you went from 120V to 116V and you can be relatively comfortable it isn’t outside influences.
Of course, any good multimeter will work for this but I like plug in meters with built in N-E measurements. This one is cheap, and the N-E may not be hyper accurate, but it is the only device I’ve found on Amazon that will show you both the H-N and N-E voltages at the same time.
The nice thing about any plug-in type voltage meter is you can watch it over a couple of days without hand holding probes in the socket.
If you find another which does both please post.