If I install '20A dedicated circuit'

Power conditioning stuff still helps. In my experience, anyway. That doesn't mean you should reconsider the dedicated line, though

Like most things "it depends!" Primarily on the quality of the AC supplied by your power company. Dedicated circuits cannot eliminate voltage fluctuations (not good for tube gear especially, because the bias current is affected), nor can they (without additional hardware at the panel) provide surge protection.

And if for some reason you wish to use balanced power for your front end stuff, you'll still need a balanced power unit.

Start by checking the wall voltage at different times of the day and night on weekdays and weekends. You can use a simple volt/ohm meter and record the readings. That should tell you. You certainly don't want the voltage to vary more than say, from 119 to 122 volts.

Once you

Can't hurt to have both, IMO.

The insidious thing about "dirty" power, at least in my experience, is you don't know you have dirty power until after it's been conditioned. Elements of dirty power...edge, grain and hash in the music...can't necessarily be heard, but are more often felt. Sometimes the presence of these elements just makes me feel uncomfortable, and make me want to leave the room. The effect of dirty power is almost subliminal, and can be analogous to working in flourescent lighting versus incandescent lighting...if that makes any sense

Why? Because dirty power can't always be heard as clearly as its absence can be experienced. It's hard to "hear" clean power because it's like trying to listen for something that isn't there, but clean power can be attributed to a blackness in the sound, and the ability of the listener to relax and settle in with the music.

The only way to experience this is to experiment with power conditioning products by installing them and living with them in the system for a few days before taking them out.

I have often found quick A/B'ing of power products to be inconclusive.