Darkmoebius,love your set-up,BTW.You seem to be following a very similar path as I did,when I tried integrating my sub "addition" to my main speakers.My friends ALL were against my adding a sub,in fear of poor integration.However with the technology available in low freq systems,I simply had to try to add that extra bit(I wasn't looking for much)of depth.Took me 2 years of auditioning too.
My room is a dedicated one 22.5x13x8,so I have my main speakers about 8 ft out,into the room.Floors are concrete.I have my sub behind the right speaker and a bit to it's right,in the right corner.Actually it is 22 inches from back wall,and 22 inches from sidewall.My main speakers are 39 inches from side walls,when measured to center of cabinet.Hope this helps.I sit 9.5 feet away from main speakers.This placement of sub,though always in right corner took EONS to get right!I'm talking about the "Rubics Cube" aspect of crossover point,placement,and gain setting.
You appear to be on the right track,and I'm sure you'll report back,soon,that you have found NIRVANA!!One thing:try to keep the main speakers out,well into the room,with really careful corner(NOT too close to corners though,as the subs need to breathe)placement of subs.I know you have to live in that room,but Audio is MORE important than your wife or mother in law's opinion!!Right?Besides a nice piece of shiny "BLING" always helps me,to get what I want.Try it!
What you are looking for(and I'm not trying to come off as an expert,so obviously feel free to improvise)is a superb sense of true "depth of soundstage",with a deep,but not overdone,sense of weight.Believe me,if you think the subs may be set too low,but are hearing low freqs and a subtle sense of pressure in room,on certain material,you are probably close to where you want to be.
Way too many set-ups are too bass heavy.When a mfgr or reviewer,for that matter,claims that the primary feature of a speaker system's impressive sound is the "GREAT BASS",that tells me it is,usually,too much bass!Which is fun,but destroys inner voicings,and subtle fine detail.Though plenty of people love this(ever pull up to a car with a MEGA BOOM BOX car system?Think he/she cares about inner voicings?).This can also,merely be my own taste.I don't care about cracking my plaster,but I do get off on a wonderful sense of inner detail,and soundstage depth,with a subtle underpinning of low frequencies;and to answer a suspicion by Raul--My crossover of 24 hz,does NOT impact anything above 40 hz.Careful set-up (not specs)has seen to that.Believe me,I would not want to go through it any time soon,but I am a fanatic.Guilty,but not sorry!!
Let's face it:when we are told a sub set-up will enhance the main speaker's midrange,it seems to me that the main speaker's mid is suspiciously lacking something.Usually the mid thickens up,when the low bass is too much,or even just a bit too much;but then there is a slight loss of subtle fine detail.It has to be sort of like Goldie Lox's porrage--JUST RIGHT!Then there is the magic,which I'm convinced you will have,as you have a "WORLD CLASS" set-up.Best wishes!!