To issue broad decelerations that one type of design
is better than another is almost always wrong.
It is simple physics. Sealed enclosures tend to produce tight, accurate bass
with a flatter frequency response curve. They are also generally the enclosure
of choice when looking for sound quality. Ported designs are more efficient
and give you more SPL output - they are generally the choice for higher
output when sound quality is less important. I explained why above but I'll try
further. More technically speaking it has to do with system Q....a ported
design tends to have much higher Q (underdamped) than compared to a
sealed design, again this is physics - the sealed box acts like a shock
absorber and dampens the movement of the cone - this makes the woofer
stop quickly when the power is removed whilst a ported design will just
waffle around even when power is removed. (Of course you have very sloppy
ported subwoofers with port tuned at 40 Hz and much better sounding ones
like the one I showed in the link above which can be tuned at 10, 15 or 20
Hz. Nevertheless, in a sub, the ported designs are always sloppier (higher Q)
than sealed - although a 10 HZ tune will have much better sound quality than
a 20 Hz tune)
You cannot ruin the lower midrange with a sub if you
cross it over properly and run it at the proper level
I guess it depends on your perspective. Most subwoofers typically add 20%
harmonic distortion anyway. There are some measurements on REL
subwoofers on the HT Shack website - so you can compare their
performance to other subwoofers.