Gulfo, Ethan, thanks for the informative replies.
Let's see...
I don't have any subs at present. My current monitors are -3dB at around 37Hz, the room is well treated for both bass and primary reflections (actually sounds pretty good).
Existing monitors are free standing towers, dipoles (I know, those in the know want us all to use point sources, yet I'm using a line source). Let me at least try it first and see, ok?
I expect due to the monitor type I'll have to be extra careful about dealing with the rear wave of each monitor, in adition to the issues already mentioned with additional reflections.
Room size is around 9m long by 5 or 6 m wide.
From what other mastering folks have recommended, I'm considering going with dual subs. There was a suggestion to try out opposite corner placement (which will need to be verified through listening and measurement).
Ethan, does that mean I need to tune the subs response to both the speakers and to the room/nodes? Through both placement as well as crossover and level adjustment? Phase as well this far down?
Would a surround room without subs (but with response say down to around 34Hz or so) be usable, or is 20-35Hz an important part of the equation (for music, I expect film requirements will be different).
Thor