Some good quality ply, 2x6's (possibly 2x8's), and some 2x4s. If i can go back and re-do it, I would of used a Kregg Jig, and made pocket holes throughout the desk to hide the counter sunk screws on the desk and speaker tier.
I did not create these plans, I used a DIY blueprint and customized the height and length to the specific room I was putting it in.
Here's the link: http://www.homestudioguy.com/PURCHASE_PDF_PLANS.html
Very detailed plans for $14.99