Garland,I should know this after a year plus on this forum, but have you exhausted all usable speaker/listener locations (esp. near corners)?
I wonder if you have a room cancellation prob. I have 4Pi's and I can do bad things to my house in the 20's of hz (broken knick knacs too). I have over 400 CD's and only two of them leave me wanting (very nasty techno ambient bass drops). I can't imagine much better bass drum than I get now, there just isn't much musical info lower than 4Pi's deliver well. Maybe there is a simple solution for you. I do run an Adcom 5400 so this may be apples to oranges.
Have you tried the Adcom in place of the Heath to see if the drums are kickin'? I think the Heath would have enough grunt with 4Pi's, but I'm no amp expert. Could the Heath be Hurting kick performance?
Do you have a quality active crossover? They help immensely with adding a sub to big ported speakers (maybe bad to put in your sweet signal path).
If you don't like active, Some swear by NOT using a high pass on the main woofers (Olsen Ariels). These folks also plug the ports where present. I do not like this, but it was better to me than unloading overlap. You have a speaker that can be plugged. You might need to, without a high pass filter you could have 2 ported 15's unloading below tuning and overlapping with the sub, likely not in phase. A last resort if things get ugly.
I have had some subs and the 4Pi renders them obsolete in my room. Neither Velodyne 1200S nor the NHT 1259 home brew could keep up with the Deltas. Maybe deeper bass, but boingy like a big rubber band and so quiet (85 dB at 20Hz was a good score just before coil hit the back plate). Seems big excursion small area is great for wet/boom sound, bad for dry/kick sound. I like the slam of the short stroke/big piston motor. I would love to hear the low tuned JBL 18 in my setup.
A big JBL with a good active X-O, Wow! It might not play a lot deeper, but with an Adcom 555 mono on it, you could feel bass at the neighbors. I know loud ain't yer thing, but it could overcome nearly any lossy room with sheer force.
Not trying to be negative in this post. Just that I have had some of my worst issues from subs. With different room, gear, ears and taste. No telling what you'll find. Can't wait to hear what comes up
Thomas,