I agree with James you really want to keep your mixing position where it
currently is and look to deal with the frequencies causing you modal issues.
appears to me that there is a very high chance that the relative flat response you are
observing at your new mixing position is a result of sound waves cancelling themselves out
as they are reflected from the back wall and the wall where your door is.This could result
in you potentially over compensating for certain frequencies including your low end
unknowingly and affecting the portability of your mix. The only way to test if this is the
case is by carrying out a mix of an audio sample you know well and listening to it in
another room.You could of course keep the door open which is helping because an open door
is a perfect absorber and doesn't reflect any bass waves but as you mention this is not a
productive set up.
In my article "taking the boom out of your
room" I suggest dealing with bass issues in three steps,
1- choose your mixing
orientation - Speakers firing down the length of the room
2- Choose your listening
3- Isolate your speakers if they are not effectively decoupled and bass trap
If you were to go with the old mixing position I would recommend you
consider trying out having your listening spot (i.e where your head will be when you are
listening to your mix) at about 1.7 m from the wall you are facing.
monitors should be placed so that the front grill is about 0.85m from the wall you are
Proceed to bass trap your room front left and right corners top to
bottom. In your case I would attack every corner with fibre glass including those at the
back end of the room. 100mm of 96kg/m3 fibre glass has an absorption coeffient of about .8
at 100Hz so should serve quite well in helping reduce the significance of those
troublesome bass frequencies.
Once you have nailed your low ends you can then
work on treating your early reflection points to refine the sound to your taste. Once done
you should be able to have a room that has great acoustic characteristics and is quiet
enough to work in.
I hope this helps with your dilema, please shout if you need
any further help or clarification
David Hyped Up Acoustics