Are Monitor/Subs More Room Flexible?

Yes, more flexible than floorstanders for smaller rooms and, very important, really full range. Subwoofer integration has been practically non-existant for years with REL subs and especially now with the availability of digital crossover and/or room correction hardware from DEQX and TacT and software from Inguz.

I would vote for #1: just get a sub. I wouldn't want to jeopardize your satisfaction with the Lineaums' mid and treble by replacing them. It sounds like you've already found a great fit with your room there. Some investment in time and software to integrate the sub(s) well might be worth while.

Vladimir, you say "Subwoofer integration has been practically non-existant for years". I take it you meant to say "integration problems" or some s

Bdarcus, you're right. I meant "problems", sorry for the botched grammar.

I do not disagree with Vladimir that Rel subs have had such an audiophile sonic response that their integration is never boomy nor gross...yet it is possible to create a small dip, Rel or not, depending on your Main speakers cross-over slope and Rel's.
I think the other thing is to know your room response and try both. My older audio room was a living nightmare for full-range speakers and I never managed to get it right, way too many nodes and stranding waves where speakers made sense. Monitors and sub were much better most of the time.