Well, isn't that what you want to do, in this case? The whole point in bass reflex is that the backwave off the driver comes around and interacts with the front wave, so you try to do it in such a way that it either negatively interacts with the natural hump in the driver's frequency response, or positively interacts in the region where it's weaker (this latter being the usual case).
A round port (I believe) means it's tuned to a specific frequency; a slit means it's tuned to a range of frequencies (someone who knows the science better should correct me if I'm wrong about that).
So short answer: something dense -- either closed cell foam or felt. Felt is easily obtained (and cheaply, I might add) at most craft stores; I don't know where you'd get dense foam, other than paying through the nose for it at a music store. (I'm not saying you can't -- I just don't know.)
You can also try varying the amount of the slit that you cover up. I.E. only stuff 50% of it, 25%, 75%, etc. Yes, I know, then we're back to how much of the backwave you let through -- but you have more control over it this way.
Definitely don't use the small speaker setting, that'll ramp up the bass. You might try some of the setting that HeadphoneAddict tried.
I'm a bung believer -- I use the bungs in my Special 25's, then corner load them. And then listen to them in the nearfield. It's a pretty exquisite experience. Other people might call it bass heavy. Heck, even I'd call it bass heavy, but it's tight, not loose. I don't have a decent preamp, but if either you (Justin) or Colin want to bring a preamp, I can demonstrate. (Just warn me, so I can set them up beforehand.)