What limits the low-end extension here?
The low end extension is limited to just how low the 5" Eton can physically play.
but if you had the option of floor-stander internal volume, what are the trade-offs between F3 and sound quality?
There is a transition area between sub-woofer and main speakers that is often just not hit very well.
If the sub-woofer is asked to play to high it get directional and really a pair of subs are needed, and they must be placed near the main speakers to make a smooth blend of this transition. I feel like this is a concern anytime a sub is asked to play above 70Hz.
Now if you force a small driver to play to low then two things are a concern.
First if it is asked it to play to low it compromises its effectiveness to cover higher frequencies and distortion levels rise.
Secondly, lower frequencies need to be covered by a driver capable of moving more air than a 5" woofer is capable of. Anything below 50Hz is really pushing it.
I imagine it'd be hard to make an active sub-woofer in the lower chamber of the speaker as I mentioned above for OBF?
With a low crossover point of around 50 to 55Hz it is not hard to make the two blend nicely, but the best place in the room for your sub-woofer may not be where you place your main speakers.
But, believe me, the Criterion's bass is superior in quality, and my 40hz test tone was played back surprisingly strongly.
The Criterion was tuned to 57Hz. An impedance check confirmed this.
In the pair I sent around I had laid in a piece of Blackhole 5 on the top of the box that should slow down the air flow around the port and minimize port noise. After installing the Blackhole 5 the impedance check showed a tuning frequency close to 60Hz.
http://www.gr-research.com/lucidity/imp.htmThis should cause the -3db down point to hit 55Hz.
They are -6db down at 48.9Hz and -12db down at 40Hz.