Building a house

Maybe  a bit more than  you need...maybe not.
And a bit of a different problem
I'm an architect doing larger scale projects, namely Educational. Often the program involves a Music Room....an instructional music class for instruments. Sometimes to listen to recorded music  as well, but not always.
This is different from a home in that has to put up with a certain level of use that you won't experience.
The construction is meant to isolate the space from other adjacent spaces. I have  done suspended and isolated floor systems, as much to isolate it from adjacent spaces but also  from  the resonant nature of the structure.  Ceilings also are a special consideration with very specific suspension methods.
At the front of all of this are the room proportions

A great way to block sound transfer in addition to the the 2" x  6" staggered studs, is to put a layer of mass loaded vinyl between layers of drywall, or at least between the studs and drywall.  And it's not that expensive. 

Decide where your gear will reside and run in wall speaker wires to other rooms so can have many places to listen and compare not just one.
Also have all the walls and floor and ceiling, as applicable, of your main room insulated ( internal too not just external walls) and install solid wood doors not hollow there so you can have some volume at night and not disturb the family.
That’s what I did. Provides a lot of flexibility for enjoying your hifi in more rooms and any time of the day

Basalt!

I know the guy who actually started his house construction with basement as a listening room and everything else was built on top of it just as an secondary addition... can you imagine six layers of different sound benefiting materials in the basement floor!? You could say that the guy is crazy, but the think is real.