How low can PRV 5MR450-NDY go sealed .

Most TV speakers are smaller and lower powered than this. Some old/deaf people watch their TVs so loudly they keep their kids and neighbors awake. Therefore, you should be able to hit 'reasonable' levels pretty handily.

Hornresp is good for this sort of question - just model a sealed back and horn size of 0. Hornresp has some ability to show the effects of filters, for example you could model a 5litre box, and 1st order lowpass at 200Hz, to give an idea of the tradeoffs, SPL limits)

"Given the high power handling capability of this driver"
--> when equalising for extended LF, power handling doesn't matter*. Almost everything is xmax limited.

A) 50watt, 86dB efficient 5" driver with 2mm xmax
B) 200watt, 93dB efficient 5" driver with 1mm xmax

using eq to make them play flat down to 50Hz, driver A will have more output than B.

* most exceptions being compact high powered subs, where $ and power are used to substitute for small enclosure volume.
 
I have used and tested this driver extensively. It works great as a top in a FAST or as a horn loaded full range above 200Hz. The fs is 170Hz and the suspension is one of the stiffest around. Look at the Vas, tiny value meaning small volume expected to provide a stiff restoring force. My sims show it can go down maybe 100Hz as a ported. But sealed is closer to 200Hz to 250Hz. So no bass. What you do get is very clean low distortion though. I like the sound and can recommend for FAST. I guess the question is with EQ can you get more bass? There is only 2.5mm xmax so probably not much more you can get. That's an interesting experiment to do a Linkwitz transform on it and see how much it can be pushed.

More here:

http://www.diyaudio.com/forums/full-range/259293-prv-5mr450-ndy-fast-applications.html
 
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There is some simple math one can do to determine how much output is possible with EQ before hitting Xmax, but I have no idea what this math is, hoping you do ?

I was thinking of a FAST since reading that thread you suggested, but would prefer to go sealed rather than ported for best transient response and don't want the size of an MLTL or horn. I thought a simple bucket sub (Subwoofer) would fill in the bottom but doesn't it still suggest the top should be able to reach down to 100Hz ideally?
 
Just use a simple program like WinISD and use sealed box. Start increasing drive voltage until xmax is reached, now look at the SPL at the frequency of bass extension you want. This is the max EQ the driver can take - then find out what the equivalent voltage is to get the same SPL at the efficient max circa 1kHz. Basically de rate the driver's upper frequency response to match the less efficient low end.