I made a 2.1 cu.ft. after displacement sealed enclosure for my rl-p 15 that I heard someone recommend. It does sound really good but I am unsure if i should have used polyfil as well and how much? thanks

You can add some if you want, I'm not sure if the 2.1 cF box is the .707 Qtc alignment for that driver or not, but a little polyfill won't hurt things. Add to taste.

so there is no way to tell how much to add? what should i start out with sealed is new to me, but sounds so good on everything.

You don't need it. Adding polyfill, to a point, will make the driver behave like it's in a slightly larger enclosure, so a heavy use of polyfill can help if you're under the recommended volume. 2.1 is right on, so I don't think you need it unless you're unhappy with the low end extension...the "rule" is something like 1 pound per cubic foot, but that's not set in stone.

id say it does a pretty good job of reaching the lows maybe ill just leave it, thanks for the info.