Replacing generic RECEPTACLES

" but if there are appliances on both legs why would it matter. Curious.... "                        
It matter due to those sorts of devices adding noise to the AC waveform on the line. The noise sort of wanders around if it is on the same line. that is why separate lines are a big thing to audiophiles. Now the noise may even get there anyway, and the power may already have a lot of noise from outside your home. It is just trying to cut down on the garbage in the AC power.

@elizabeth is exactly right.
To take this concept further, the ideal setup of the service panel for cleaner audio would be to have your dedicated audio breakers grouped together at the top of the panel. Or at the bottom, if that's where the wiring enters.

So, if the lines enter at the top, your audio lines would be in the first breakers, same leg. Then the heavy duty appliances that you listed would be far away near the bottom split between Legs A and B. The theory being that the noise introduced into the panel would not be close to the audio breakers.
But, since the grounds and neutrals are shared, noise will still contaminate the rest of the panel to a lesser extent.

To take noise reduction even further, many audiophiles use a subpanel for their audio lines. This is a smaller dedicated circuit box attached to the main service panel. 
When I expanded my system (which is very modest), I received step by step advise from our most experienced members on how to do this in my house. Now that I have a subpanel which has it's own ground and is tied to the service panel, the noise floor at my audio setup is extremely low. It helped in my case because I live in a prewar row home with a mix of old and new wiring.

If you had all new electrical installed in your home and the distribution of current draw is laid out well, you will be in very good shape with 2 dedicated lines. If you run that 2nd line, move it close to line #1 on the same leg.

@elizabeth @lowrider57 ............Thank you both very much for all you replies... It really helped me see and understand so much more about electric wiring  and breaker boxes.. Now when i looked at my breaker box my quad 20A breaker for my amp and preamp is on the bottom of leg 2. When my electrican comes and installed my other 20A dedicated line for my CDP should i tell him to relocate the 2  20A dedicated audio lines to the top leg 2 of the breaker box ?

correction ...Not quad 20A breaker... quad 20A receptacle. 

I do not know if moving the wires in the box would actually matter. Sorry I cannot advise you on that.