Hello all!
I have been studying and reading and learning for a couple years (on this site and others) about how to build a soundproof (I actually prefer the term "sound reduction" now) room in my basement.
My band plays LOUD. Like jet airplane loud (120 dB+ at times). Yes we wear earplugs.

So I finally went ahead and did it. I built the room.
Your mileage may vary - so take this only as one anecdote of supporting info: the advice on building and materials you are reading in this chain basically worked for me.
The sound of the kick drum and bass is now only BARELY audible if you stand very still near the basement wall and listen for it. From standing at the neighboring houses (which are pretty close - like maybe 12 feet away) you cannot hear it. Basically - I cannot measure the difference between normal city sound outside my house and my band playing. This is a very good thing. Not perfect - but very very good.
I will try to give more details about what I learned in this process and what a "normal guy" building a room runs up against - but I must say - I followed this design and it works for me.
1. Built a separate "floating" slab that does not touch the basement floor or foundation
2. insulated the ceiling of that area of the basement with the bonded cotton insulation
3. covered the cotton insulation and ceiling of that area of the basement with 1 lb MLV (attached to the floor joists above with a cap nailer - works great!)
4. built a free-standing room on the slab with its own walls and separate ceiling (does not touch the basement ceiling above!)
5. covered and sealed that free-standing room in 2 lb MLV (cap nailer again!)
6. put the closed cell 1/8" tape (2 layers) on every other stud (on top of the MLV) - to give a little air gap for the MLV to stay limp
7. 1 layer 1/2" drywall (horizontal)
8. layer of green glue
9. 1 layer 1/2" drywall (vertical)
I took measurements of drum kit and guitar playing at every new layer of mass, and have the numbers if folks are curious. Real world SPL numbers for each layer. Also I have the actual subjective experience of hearing what each layer did and will try to share it here eventually if folks are curious. I also took pictures along the way.
At first I was really surprised how little the last (and most expensive!) layer of drywall + Green Glue was seeming to do. I really expected a big jump in the sound reduction (especially the bass) with that extra mass and the high tech glue layer - but I wasn't seeing it on the meter - and while I could hear a difference - it wasn't as monumental as I'd hoped.
But after about 3 weeks suddenly the Green Glue must have "set" or something (the packaging does say wait 30 days - but at the prices Green Glue costs you really want those immediate results!). The difference was pretty amazing if you ask me. Suddenly the kick drum and the bass were tamed! Awesome!
My take - don't give up on the Green Glue - give it time to set and it seems to really (and I mean really) work! It was like night and day for the bass frequencies in my case.
I'll post more about my experiences if people are curious - but my simple advice is that the ideas listed here seem to work, and that the devil is truly in the details with sound reduction. Seal every crack, tape every seam, and don't skimp on materials (but apply your funds judiciously in the right places - MLV, Drywall, Iso Tape and Green Glue).
Mass-Air-Mass, variation in materials, and keeping pieces "disconnected" from each other all helped for my room.
Just a general info post and a thank you to all the advice I read here and everywhere in the sound reduction community!
Thanks!