Feedback Delay Network for Reverb

A = (a + b) + (c + d) + L
B = (a - b) + (c - d) + R
C = (a + b) - (c + d)
D = (a - b) - (c - d)

Max feedback should be 0.5 at the very most to avoid nastiness.

Each delay line should have a delay time that is not correlated to any other delay line's time.
Can use golden ratio or other irrational number:
  A_time = time
  B_time = gr * A_time
  C_time = gr * B_time
  D_time = gr * C_time

golden ratio = 1.61803398875
1 / gr = 0.61803398875

Better yet, slowly modulate the various delay line's times using complex lfos to avoid coherence as well as adding some nice pitch instability :)

Cascading and summing multiple delay networks can yield some very nice results.