Hi mark
I don't usually post here but i thought i could help you here cause i've been
studying room acoustics a lot lately.

ok

first off
your speakers should ALWAYS be positioned to the lenght of the room rather then the width.in your case they should be up against the 9 feet wall
(depending on what monitors you use , you might want to move them a little bit away from the wall).The reason for this is bass reflections :
since low freq waves (bass) are much longer then high's , reflections of the
walls cause phasing and bass cancelations (which i think is your case), thus
you need as much lenght as you can get.

Second thing is room shape.You'll need as little standing waves as posible
so you should have something to brake off paralell surfaces.but since you
already filled the room with records and equipment there's not much you can do about it.what you can do is to put things(gear,couch,whatever)
in random places so that bass can't reflect more then twice(floor to ceiling).

Next problem is corners. Bass frequencies act funny in corners and actually
increase in amplitude when reflected, so what you need to do is to put bass traps in 2 corners of the wall behind you.
These things have shape of a tunnel and are placed up against the corners. what they do is ,they kill some of the bass energy and stop it from coming back at you.

umm...yeah

Any diffusion is good and don't put to much of absorptive material, although some is ok.

And for the room tuning you'll need a RTA( Real Time Analyser), pink noise generator and a calibrated mic.But room tuning is a lot more complicated then it sounds.
For that I would hire a pro audio engineer if i were you.

.. i hope this helped ya somewhat.