Sub Integration

Thanks Bruce,
You've offered some very good suggestions. A couple more questions that maybe you and others can weigh in on......
The PSB Imagine B speakers are rear ported and PSB suggests that the closer the speakers are placed against the rear wall the more enforcement there is in bass. Mine are placed about 3 feet away from the wall. PSB offers port plugs for some of their floor stander models but not for these. I have considered plugging the ports and port plugs are offered in a generic sense on the Internet but I wouldn't know which and what size to order. For an initial experiment on the results of plugging the ports with the design you suggest, I have an abundance of foam of different sizes, much like sealing up openings around a window air conditioner (soft foam). Compressed and sealing up the ports, would that suffice for an interim port plug?
The Outlaw ICBM that I use for bass management recommends to set the crossover frequency on the subs to their highest frequency and then let the ICBM determine what frequency the subs actually see. Does that make sense?
The NAD amp and the PSB speakers sound very good. I originally had the speakers connected to a very good HK receiver. The sound was horrible. Like many have said that there is some truth to NAD and PSB having some chemistry since they are, or were, sister companies.
So far I have received some great advice from you and others. Will continue to obtain sound that is as good as my system is capable of.
Thanks All

You should look into using the 1/3 rule especially if the speakers are rear ported. 1/3 rule is to move your speakers 1/3 of the room length from the front wall and move your listening chair 1/3 of the total room length from the behind the chair. This is a common practice as a starting place for the speakers. Also, don’t put the subs in the corners, never do this unless your designing for a home theater. Move the subs out into the room and you will get more even bass. Check out Jim Smith books/videos or the Cardas website for better clarification 

I’ve run a test CD to see where my main speakers "actually" roll off in the bass realm, as opposed to confusing "in room" compared to "anechoic" ratings, or actually misleading published specs. You can use a test tone to reveal room bass issues without the expense of some digital EQ gadget. Your ears might work! My main speakers (Silverline Preludes) are allegedly capable of 35 hz or something but really 50hz is where they drop off (you can hear there is a teeny bit of 35hz coming from these things if you put your ear an inch from a woofer, but hey…nonsense)…consequently I use 50hz as the starting point for woofer settings (2 REL subs) and that works swimmingly. Also, I recently committed audio heresy by getting a Schiit Loki 4 band EQ and I can now highly recommend these…an el-cheapo and utterly noiseless little gizmo that really does it’s thing. I only use it when I feel a source sounds weird or when I want low level sound tweaked…like a "loudness" control…the 8k higher level knob is a serious cymbal booster and the 20hz low knob will tame bass issues. I doubt many people can actually do the 1/3 rule thing as often they have to live in the place where the audio rig resides, but there clearly are other solutions.

The 1/3 rule suggested by rbstehno is appreciated but does not apply to my listening space. My room is 15' wide and 20' long. If I apply the 1/3 rule then my listening point would be only 8' away from the speakers and in the way of casual traffic in the room. Not practical for my space. I really appreciate all of the suggestions offered and will continue to experiment with all of them. Perhaps I have a very challenging task.
Thanks

I would crossover your sub naturally in the nighborhood of 50/60hz, where you hit the -6db point from your speakers.  This will take care of the boominess.  As for the volume level,  you can use white noise and music and do it by ear.
Most of the music is played at above 50hz, so there's no need for sub above that unless you like fat low bass.  To me, a sub is meant to be used for sub-bass (40hz and below).