This concept seems hard to get across: muffled = softer. That's why they call it muffling.
With a big port and lots of muffling you won't get as much volume as no port/no muffling. You can get "thud" or you can get "boom" but it's difficult to get both.
If you play out miked--or if you just want this sound in your practice room--you can go for "thud" and the PA will make the drum loud enough. If you play out unmiked, though, you really need some sutain ("boooooom") for the bass drum to be heard.
We play mostly unmiked, so I use (as Jsk36 above does) PS3s front and rear, no port and nothing in the drum. For miked I use a PS3 reso with a small port and an Evans EQ pad on the batter, and possibly a small rolled-up hand towel resting against the reso.
To get a little more volume and boom from your setup without getting new heads, try this: First, take everything out of the drum. Then starting with the reso, bring it up very gradually from finger tight, no more than a 1/4 turn at a time. Strike the head in the middle at each stop. At first it will sound flappy/papery. Keep going until you get a real tone, one with some bottom-end and sustain. Stop there. This is above the "just above wrinkle" stage, but not by much. Then tune the batter the same way, except make it just a little tighter. (The more you tighten the batter the more you reduce sustain and increase attack.)
This will give you maximum boom with what you've got. Now's the time to add muffling, just bear in mind if you throw blankets and pillows in there you'll reduce both volume and boom. You might want to start small, with small rolled-up towels resting against the heads.
Just as an aside, the SKII is to me a head that is overly muffled. For my purposes it simply isn't loud enough. Other folks have other opinions, so YMMV.