place them in pretty much identical postions within the room, will the suckout remain?
More or less - YES (changing the height of the woofers will only affect reflections from off the floor and ceiling - the excitation of one mode only).
Does the room have the final influence on reproducing the frequecy or can it be overcome with a different delivery from the speaker?
No - you can actually change somewhat the severity of suckouts and peaks by moving the speakers around and changing your listening position (affects two modes). Don't place speakers in the corner and don't sit in the exact middle of the room, for example.
However the fundamental room modes do not change as they are related to room dimensions that cause reinforcement or cancellations. What you change is the degree to which they may be excited and the degree to which they are audible.