I am a fan of ported.

Yes, what he told you was way off. With ports so short, your box is tuned too high, causing a frequency spike in the upper bass range (my guess is around 80 hz, but that is just a guess) which may account for the "boomy" sound you have talked about. Before making a sealed box, try extending the ports to 15"

You were given specs that tuned your box to crap. Before canning the ported idea, try a correcting to a proper ported.

If ported is done wrong, you'll think all ported sucks
If ported is done right, you'll know that ported rocks.

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