Is anyone familiar the the Adire Audio Tempest 15?  
I am able to get a few at 139.00 CAN and wondered how they (2) might work in a HWK 15
With a 350 WPC plate amp.
T/S
4 ohm     Fs 18.8     Qms 6.7     Qes .40    Qts .38   Vas 317 liters  Xmax 16.4mm
Sd 779    BL 14.2    Re 3.5     Le 2.9     SPL 89.1   Pmax 750 watts
Diameter 15 3/16
Depth 8
Cutout 13 13/16
Weight 21lbs

I am looking to get down to 16 Hz if I can. I was thinking of the HWK 18 with one 18, but I was rereading the suggestions Steve has for the HWK 15. I am now thinking of using 2 15" subs with both facing up to the large port Steve suggests that this will bring the bottom end down.  Trying to keep with the parameters that Steve suggests, I have come up with the Adire Tepest 15, and 3 Dayton 15" subs that are close to the 10 cu ft vas and the "around 20 fs" as seen below.  
I have been flip flopping around regarding making a decision and need to get going on this one.

I need 16Hz-about 90 Hz  Pipe organ music mostly. Is that crossover point correct when the mains are 45-20,000 +/- 2dB?

As steve suggests regarding the 2 subwoofer position that I was looking at.
"This is a take-off of the first alignment that tends to shift the response down by 10 or 20 Hz and is considered a true isoberrek alignment.  The large air spring between the woofers yields interesting results with different woofers.
and the less that .38 qts"