common enclosure with 3 8 ts sw841d's

I have 3 Pioneer Ts SW841D's (4ohm) wired in parallel and mounted in an undivided sealed enclosure with a total of 0.84 cubic ft without the subwoofers installed.  I am unsure of each subs displacement volume.  I listen to mainly pop and rock and the subs have a muddy sound and tend to over excurse or possibly bottom out at mid volume with minimal bass applied. At low volumes the sound is pretty good.  I am wondering if my box is too small?  I would not think so since the recommended volume for each sub is 0.15-0.50 cubic ft.  I have also stuffed the enclosure with poly fill.  I removed the poly fill yesterday and it sounded worse and therefore I put the poly fill back in the box.   Would it be possible to port the box and get better results? The box is wedge shaped with internal dimensions of 6.5" at the bottom and 1" at the top with a height of 20 inches.  I am running a kicker 500.1 on these subs with the bass boost set to 0.  I am looking for a solid kick drum without the boing, boing affect. lol Thx

I figured out what my problem was.  Somehow the bass boost on the head unit was turned up. Once this was reduced to 0 the problem went away.  I still would like to know if porting the box is a viable option. Thx