Dedicated Power Line Project

I’m going to be adding a dedicated AC line to feed my audio system. Currently, the system is fed by a 75 foot run of 14 gauge romex that is likely daisy chained a few times in the wall between outlets. Needless to say when I turn the volume up, dynamics suffer and the overall sound quality is much less than I know my system is capable of. At louder volumes, bass notes will make the lamps dim and if I go louder, it seems like the system struggles and the bass notes get rounded off or sound “fuzzy.” My calculations tell me that with 75 feet of 14g romex, the voltage at the end of the line could be dropping by 5-10 volts, maybe more, during transient peaks. What I think is happening is that the power amps’ power supply caps are draining and there isn’t enough current to recharge them quickly enough and the amps start to draw directly from the AC line which causes a large voltage drop. 

Initially I was looking at a 10 gauge or 12 gauge line with a 20 amp breaker. After running the numbers, even a 10 gauge run could see some significant voltage sag at 75 feet during transients. At that point I considered 8 gauge, but the problem with that is that no electrical outlets will accept anything larger than 10 gauge, so I’ll have to do something fancy like place a junction box and step down to 10 gauge at the breaker panel and just before the outlet. Then I thought if I’m going to that much trouble with an 8 gauge, why not run 6 gauge? My calcs tell me 6 gauge could have a volt or two less variance than an 8 gauge and it wouldn’t be much more cost. So as of now, the plan is to run a 6/2 solid core romex line and run all the equipment of that. (I could run another line for the front end in the future but that isn’t the plan right now).  I have a few questions if anyone would be so kind to reply:

1) Are there any foreseeable issues with running “too large” a power line? I’m not sure if it’s accurate, but I’ve heard that large gauge is good for bass and small gauge is good for treble. Since I’ll be running everything (including the low current draw components like sources, preamps, etc) off this one line, is there any chance I’ll change the sound signature in a negative way?

2) What is better? In-wall romex or THHN inside a grounded conduit? I’m leaning towards romex because my gut tells me having the wire surrounded by something could hinder transients or hurt clarity but I’m not sure that’s accurate - that may just be from my personal experience with low level signal cables where too much insulation can be a bad thing. 
3) Are there any other “gotchas” that you can think of?

I’ve read the MSB Technology article and I’m subscribing to that theory which is this in a nutshell:

“It is the subject and goal of this paper. The gauge of the wire is FAR MORE IMPORTANT than the fact that the line is “dedicated”. The subject of this paper works on the theory that the varying musical demands of your amplifier are actually modulating the incoming power line, divorced from the utility (power company) by some resistance (12 or 14 gauge wall wiring at some length from the breaker panel has too much resistance for audio purposes).”

Thank you and wish me luck!