I received my box today. It's sealed 1.25 cubic ft. per chamber for my Diamond Audio D612D4 subs. It has 1.5 lbs. of poly-fill per chamber as was recommended. Each chamber is approximately 90% full with this poly-fill.
Is this too much? I've used poly-fill in a few boxes in the past, but never this much.
You should be just fine. But probably the most important thing is to experiment. Over the years I've been installing (15 and counting), Ive always heard and found out preatty much the same thing. Poly fill basicly makes your woofers sound like they are in a larger enclosure than they really are. The combination of your woofers and your box will sound different in every car you put them in. That combination may sound killer in one car, and weak in another. So try an empty box, then try a filled box, and then mabey try a half filled box. If you let different shops tell you what sounds good, you could end up buying their products (thats what they want), so do what sounds good to you.
As for the car audio shops, I don't listen to anything they say. I've heard way too many lies from salesmen at car audio shops. I've used poly-fill a few times in previous boxes myself and my friends have had. I've just never filled the chambers full of poly-fill like this. I had my box built to the specs recommended here and at another forum and shipped from an online box builder. The 1.5 lbs. of poly-fill was recommended here. The issue that slightly worries me is the fact that my subs will have to squash alot of this poly-fill down because the chambers are almost full. I guess I'm worried that it might interfere with the mechanics of the subs. I'll experiment, though.