Sound dampening

I'm helping a friend with his theater and one thing he is doing with a 7" high platform for second row seats is lining the platform with insulation and adding port cut-outs that turn the whole platform into a giant bass trap.

You want to break up the sound that travels along flat surfaces. Port the platforms as you described but fill with random stone and not fiberfill. Bass waves will still pass thru the fiberglass. The stone will diffuse and reduce the chance of standing wave resonance. Tom

Bass frequencies will not be damped or absorbed by curtains if that's what you're looking for. And they won't do much to keep the sound from leaking to the rest of your house either if that's a concern (you don't specify if you're looking for sound isolation or optimization or both). If you're looking to just absorb higher frequencies I suppose the curtains would help but could even prove to be too much. If you construct the walls and ceiling the right way the need for additional damping will be reduced. I'd highly recommend reading the book Premium Home Theater by Earl Geddes as it will give you good theoretical and practical guidelines that could help you decide how far you ultimately might want to go with this. Best of luck in building your theater.

Spend some time, considerable time, on AVS Forum and there are thread upon thread dealing with room builds and set-ups.

I presume this is a basement? Not totally knowing how the structure is laid out and finished, It's likely safe to assume "medium to lowish" bass absorption in the space (say, compared to a large commercial cinemas space - which absorbs a lot of bass, n so lots of mid/hi absorption throughout to balance out the space's accoustics). In this case you'll need to do SOME mid/hi frequency absorbers in the front half of the room (i.e, behind the speakers, and likely one absorber to either left or right side of L/R speakers beteen speakers and listening seats), with mostly diffusion around the rest of the space and back.(taking care to treat all the first order reflection points from wall reflections from speakers to seating locations) Ceiling will likley need cloud absorber between mains L/C/R's and seating (if mostly movies, otherwise diffusor if lots of music dubties).
I'd strongly consider bass traps in the corners, which can be hidden, and I'd also strongly recommend possibly ONE LARGE CUSTOM BASS TRAP hidden at the back of the room,(you could hide with your curtains) if you don't want to touch the ceiling! (a large much needed bass trap will help the room TREMENDOUSLY, add huge impact to the room, smooth out the bass, make it faster and tighter, and the room will sound larger, if you can swing it - and I'd partially cover the trap with diffusors for the mid's/highs).
All of this could be easily covered with rods and "accoustically transparent" curtaings, with lighting peaking out between the curtains, if you want to simplify, make it look clean and theater like, and hide all the speakers, wires, acoustic stuff, etc. Sonically, THE ROOM WILL ROCK!
Also, you should strongly consider using even number of seating, placing seats at 1/3,1/5,or1/7th seating locations relative to boundaries, and placing speakers 1/6th,1/10th, or 1/14th locations correspondingly - then EQ the rest with todays modern DSP room correction.
That's all bare minimium if you want tremendous sonics, and can go for broke maximizing your audio experience!