Hiya Nick,
Glad to read of your progress on the basement in the new house. I had the same issues when I built my HT in my basement a few years back. Here's what I did:
Walls - 2x6 plates with 2x4 uprights to decouple as you have already done. I did 16" on center but obviously offset from one side to the other.
Floor - Dri-core subfloor with carpeting
Door - Sliding solid wood pocket door - not sealed but being solid it works fairly well
Insulation - I used
Roxul Safe n Sound because it was readily available and is a good sound and fire barrier. All walls and ceiling/floor joists were packed with this stuff. Double layer in the ceiling actually as there was room.
Vents - I put S bends in the duct work to help stop sound from the air vents making it into the main ducts for the house. I also used MLV mini stick on sheets to get rid of any resonance in the duct work.
Ceiling - used 5/8ths drywall in the ceiling instead of standard 1/2 inch. Also decoupled it from the floor joists by means of resilient channels (Z shaped metal strips). Using these you have to be very careful where you place your drywall screws so as to NOT put them into a floor joist - mark your drywall 1st.
I did a bunch of research and found this for me was the sweet spot on price vs. soundproofing extremes. It covered all the basics for minimal extra cost. Hope that helps, let me know if you have any questions.
