I am going to build a listening room

First, make it aesthetically conducive for relaxed listening.

Second, if you like to listen louder and are at risk of disturbing others, insulate it as much as possible.

Third, start with the right speakers that you like that will fit into that room best. For a smaller room, they need not necessarily be too large or uber-expensive accordingly.

Fourth, start simple, and tweak and adjust one step at a time over time as needed until things are tuned in perfectly.

The thing not to do is go overboard at the outset with treatments, etc. Start simple, assess what you have relative to what you want and take things one step at a time from there.

Make sure your basement won't flood. Next, bring in an electrician and discuss your power needs. finally, survey the soundproofing landscape (sheet vinyl, alternating studs, etc.) to keep the sound to yourself. Good luck!

Suspend the room on springs and make the room spherical. Paint the walls industrial green.

I'm doing the same right now. Roxul is very cool insulation. Any air gaps are your enemy. I even installed cool running can lights so I could insulate right over them (LED). Any in ceiling speakers should be enclosed, no open backs. Quietrock is very neat drywall for walls but is not recommended by the manufacturer for ceilings because it can sag. It's expensive too. Pre-wire and do your best to keep power away from signal, electricians don't seem to care. I ran some extra lines just to future proof things. For the concrete basement floor I put in a floating floor that home depot and lowes sells. It is 14 inch squares with a black plastic backing with feet. They interlock together and can be glued together if you like. It keeps it warmer, keeps any condensation from the floor away from you and helps with small water issues. Floods are a drag but not much you can do except have insurance and keep everything off the ground.

Finally, power: You can never have enough. Whole house surge protectors will stop lighting hits but not as well as you think. We have been hit twice at the pole and even though it tripped the whole house surge, it tripped the surge protectors inside. They are more sensitive and will trip faster than a big 200 amp unit. Do both.

Mike Holmes and others have a bunch of videos on youtube about roxul, quietrock and more. They are worth spending an hour on.