About the F20s....you said you were running music through them. Music rarely contains content less than 30Hz, so the F20 can take all 500W without reaching Xmax above 23Hz or so. Movies, on the other hand, will test the F20's limits.
Careful testing will reveal where those limits are. Using Soho54's Audio Test DVD's -0.5dB LFE tracks can really help, along with a careful gain structure setup.
Unless you highpass, the F20s will literally loaf all night above 23Hz, with clean bass, no compression worries. With 4 of them clustered, instant +12dB over one of them. You should be good. If for any reason you need more headroom, you can use more power and highpass for an extra 3-5dB, but then you are running them near the edge.....
I think it best to limit the content they get to 20V maximum below 25Hz, and above 25Hz to 30V maximum. 30V is 300W. You are far from Xmax, and within your amp capabilities. 30V at 17-20Hz will exceed Xmax, and may near Xlim, causing clanking and possible damage.
Remember, it is always best to run at least 3dB from limits, and 6dB from limits is even more preferable.
Good articles about cinema sound:http://www.hps4000.com/pages/articles_page_.html
Good 1st read is the "If they knew what you were missing" article.
HPS4000 really doesn't output much below 35Hz, so take that into account when reading. The horn loaded subs are tuned to 40Hz, but the systems have enough headroom that sound would nonetheless be pretty impressive. When HPS4000 was conceived, films rarely dug below 40Hz. But the articles are worth reading.
Even is you have to run below reference in practice (remember, reference level is LOUD, no one speaks at 75dB in normal conversation, except maybe Drill Instructors), running with enough headroom to present cleanly is a must.