Of course all things are "relative".
The 626 has a single neopanel and the RM30 has three.


The 626 has a single woofer, and the RM30c has 2 "plus" a pair of Passive radiators.

At very low volumes, they will sound the most alike as they share the same drivers, but as the SPLs increase (as Brian offered) the single panel will obviously have less potential. This is simple physics. 3X the midrange drivers will have higher powerhandling and the stresses of higher SPL's will be distributed over more drivers.
Now that doesn't mean the 626 cannot play loudly as it really is a pretty ferocious performer, but when you have 3X the midpanels you can pretty much blow the walls down with "clean" output.
I remember many years ago at CES when Brian was first showing the RM30's with some AMPZILLA 2000 monoblocs.
A couple young dudes cam in with some Heavy Metal and ask Brian if he would put it on and "TURN IT UP" as they took their seats.
Well B, pretty much lit up the place with SPLs so high that I, and most all the other parties (except Brian and the two Dudes) Had to leave the room.
And I remember standing outside the room and thinking how incredibly CLEAR and CLEAN the sound was at that SPL, when most any other speaker would be absolutley distorting all over the place. It was actually quite a display.
And not that SPLs are the only reason to think RM30, you will also find that the larger the speaker model the larger the soundstage.
And don't let this give you the idea that the 626 will somehow be WIMPS.
In most any normal sized room the combination of the LRC, and a pair of 626 with a sub and X-Over that will slightly reduce the low bass to those speakers you can do some pretty high SPL's with exceptional clarity.
This then goes back to my original statement, that it is ALL relative.
BIGGER can be better, but smaller can certainly DO.
As a side note I use a pair of RM30s as "side surrounds" and they are spooky good. (and likely OVER-KILL) Concert DVD's and Movies are incredible.
I might also remind you that the RM2 is also a step up (and in between) the 626 and the RM30.
