I have a couple of questions about subs, that I haven't been able to find answers to.
1. With all things being equal, what is the difference between a sealed and vented design?
One is sealed, the other vented. Really.
There is a lot of myth in the sealed vs. vented debate. How well a sub reproduces a given frequency is a direct result of the quality of parts and the quality of the design.
Where sealed subs are damped by the sealed internal air mass, a (good) vented sub relies on motor strength to start and stop the driver properly to accurately reproduce sound.
Because of the way a sealed sub rolls off gradually, it will often require some equalization (either a cut in higher frequencies, and/or a boost in lower frequencies) to give flat output across its range.
A vented sub will require a high pass filter to keep frequencies below its optimal range from destroying the driver (no natural damping as with a sealed design).
A driver designed for use in sealed enclosures will not work as well in a vented design and vice versa.
As for group delay, there are well known and respected designers on both sides of that argument and you can find some interesting discussions on avsforum.com and diyaudio.com where both sides duke it out.
What I mean is that if you have two different subs level matched, playing within their limits in the room, with very similar flat frequency response curves (from say 20Hz to 80Hz), is one sub going to have more of an impact (thump you in the chest bass in action movies) than the other?
Does the volume of the box, play a factor? Or does the amount of RMS wattage that of the amp play a factor?
Yes and yes. In general, you will need more power with a sealed design than ported to produce the same volume and range of sound. A lot depends on driver design and efficiency as well though.
Also, ported systems need internal volume to reproduce those extended lower notes so they tend to be bigger, often bigger than your average person wants.
If you want the point made with real examples go to the diy subs and speakers sub-forum at avsforum.com and search for a thread by bossobass on his dual opposed 15" sealed sub design. It is very compact and fed by something like 4,000 watts of power to produce incredible bass from 10Hz on up.
Then search for a thread on the wolfhorn a 'ported' (well horn loaded actually) design at the other end of the spectrum. It produces the same bass with a fraction of the power, but is, um, quite large.
If you skipped all the way to the bottom to see if I had a point
, you can get similarly good (sic) bass from either design, just in different ways.