Did you try different loadings and different locations of the box?
Steve said:
# This box is offered for hobbyists who love to experiment. Since there are so many ways to load it, you should be able to find at least one that works with your woofer - regardless of the type.
And also:
# This enclosure will properly load rooms between 144 sq. ft. and 3200 sq. ft. with extremely serious bass.

Using two is difficult to make them sound right together, don't do it.
Build one and experiment with driver(s) positions. There are 14 ways of loading it.

Bart

Greetings,

Just a quick question, what did you expect from the build?  
I am a newbie here, but it seems to me that many subs tend to emphasize bass for 30-40 range for HT and that good, low Hi Fi bass does not necessarily have the same "sound one expects".

I am in the process of buiding a HWK 15 with two drivers (both pointed upwards to the large port), so will post my findings. I have found the folk here very helpful and all have said the same regrding the HWK-- it is an excellent sub.

Have you EQ'd the sub?

I have experience with 2 subs in a room and it can be a nightmare to pull off and we are loading a space 24X48X24 ft (2x Paradigm 12" PDR 12).

Others out there may have much more intelligent things to say--just throwing in my 2 cents.

Fr. Ray

According to Steve, "For the 15" version ideal woofer selection if you going for low bass would be a subwoofer with an fs around 20 Hz, and a Qts less than .38 and a minimum VAS of 10 cubic feet. "


I am looking at these from the US:http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=295-190

and considering these  from Canada:call Bob Reimer at Creative Sound Solutions
604-504-3954    www.creativesound.ca  they have a few Adire Audio Tempest drivers left at 150 CAN each  
fs 18.8,  qts  .38,  pmax 750 watts, spl 89.1  VAS 10.85 Cu ft

What is your budget?

I was looking at a couple of these for a while from Mach 5 Audio in Canada for an 18" HWK build but the $$ got in the way  :(
http://www.mach5audio.com/index-1.html

Fr.Ray

I found that tidbit on the bottom, of the "white paper" for the HWK--it's in a box at the very bottom of the list of schematics on how to load the HWK.

As far as the changes when isobarik loading here is a good discussion if one can beleive the credentials of the fitst person to post:http://www.hifi-forum.de/viewthread-59-561.html
As you scroll down you will notice that VAS is halved with "clamshelling" SPL goes up, but qts remains the same--again, accoring to the aformentioned post...

Fr.Ray

Thanks for the info Gex  :)
Fr.Ray