Dedicated 20a lines, 125 feet distance, which gauge wire to run?

#1 wire is a lil over kill but, go with it. Do you really need a 150 amp panel? Just make sure the buss is Copper! I'm happy to see your going to use VH wire. Looks like your on your way. Keep us in the loop.

The power conditioner at the subpanel would work on all 4 lines, yes. I was planning to have the Niagara 7000 on it’s own dedicated line, with all electronics connected to it. And 2x MC901 monoblocks each on their own dedicated line. So, 3 of 4 dedicated lines allocated.

For me, no Niagara at the subpanel. That's just a bad idea if you have a problem, you'll need an electrician again unless you're handy. Good ideas on the wire, your electrician sounds competent. Silver plated outlets, not gold.  

For the receptacles, I was planning on using 4x Furutech GTX-D NCF(R) AC Receptacles. Is there anything better?

My experience with rhodium plating is it is very pretty but a terrible interface to another surface.

It is too hard to allow for a great grip. Copper alloys, like on a normal wall socket or spade, are soft and deform readily. This malleability makes them deform and make a more complete and tighter connection to another metal. This is something you can readily feel when making a hard to hard surface connection, like nickel to nickel or rhodium to rhodium. It never feels as tight and secure than compared to using a soft to hard or soft to soft connection.

For instance, a nickel plated speaker connector has a very hard time staying tight onto a hard spade. Switch the spade to say gold over copper or the WBT spring loaded spades and the problem goes completely away. I have also used rhodium AC connectors and faced the same problem. They just do not grip or stay in the outlet as well.

The worst high end connector I ever used was a Furutech locking banana. Maybe I got a fake one, but it was just like half a millimeter too small, and the rhodium plating was TOO smooth. It was nearly impossible to make a strong speaker connection. The inexpensive solid copper spades were far superior and a lot less expensive.

Not saying you should avoid Furutech, but all of my experience in various locations (AC/speakers, etc) says rhodium is not very good except to look at.