For HT, I beleive one sub is needed at leat for LFE. So,.....
with current speakers.... two original subs (adding a larger also for LFE would be optimal)
With RM2's, 40's or RMX..... one larger sub for LFE only with speakers set to large
I say this about the current setup as adding a sub for each speaker would be best for music (basically turning them into full range) and then the larger sub would be LFE only with the 626/orig combo set to large
If HT is you main interest though, I would think the RMX would be too much as I don't think the best music speakers are always the best HT. To get the most from these speakers I think you need a high end preamp so if you use a pre/pro you would need one with a HT bypass.