Surely, even with a sub woofer there is a noticeable difference between an rb-61 bookshelf speaker and an rf-82 floor standing speaker? The KLH speakers currently being used as surrounds are $30/pr at Bestbuy so hopefully the rb-61's will be a significant improvement!
It is difficult to tell from Klipsch's web site, but they appear to have the same tweeter:
http://www.klipsch.com/products/details/rb-61.aspx
http://www.klipsch.com/products/details/rf-82.aspx
So, the treble probably sounds pretty much the same.
The woofers are of the same type, though they are obviously different, as they are different sizes. However, they very likely have similar tonal characteristics, excluding deep bass. Klipsch did design these speakers as part of the same line.
Now, the towers are more efficient, so this means that they will play louder with the same power. But, does your system play as loud as you want it now without added distortion? If so, that is unnecessary and therefore pointless to spend money for it.
For the bass, they have a -3 dB of 43 Hz (RB-61) and 33 Hz (RF-82). Both are more than low enough for use with a subwoofer. Obviously, you could set the frequency in a bass management system lower with the RF-82, but it is doubtful if that would be very helpful. And both are not low enough that they don't need a subwoofer if you want the very lowest bass possible.
So, unless you want louder, it is doubtful if there is much of a sound difference between the RB-61 and the RF-82 when used with a subwoofer and compared with appropriate positioning (e.g., placed so the tweeters are at the same height, etc.).
Of course, you can listen for yourself and come to your own conclusion, but you must listen to both with the subwoofer (appropriately adjusted for each) to be a useful comparison for your purposes. Clearly, without a subwoofer, the RF-82 goes noticeably deeper, and should sound better. But with a subwoofer, unless you want louder, it probably sounds almost exactly the same.
With modern surround systems, it is almost always a far better value to go with a good bookshelf speaker than to go with floorstanding speakers. There really is no point in spending money on bass capability in a speaker if you are not going to use it.
In my case, I use Aurum Cantus Leisure 2SE (original U.S. verison) for all channels, with a pair of SVS CS-Ultra subwoofers for the bass. Here are links to the new versions of each of these (mine are old, no longer made versions):
http://www.kellsieavdesign.com/products/Leisure2SE.htm
http://www.svsound.com/products-sub-cyl-pcultra_new.cfm
The new versions should be slightly better than mine, though I have no intention of upgrading.
Now, I could have purchased higher model speakers for the main channels, such as:
http://www.kellsieavdesign.com/products/MG.htm
They cost more than twice as much, and use the same tweeter and same mid-bass driver*, but add a woofer for deeper bass. Undoubtedly, without a subwoofer, they would sound better than the Leisure 2SE. But using a subwoofer, they probably sound almost exactly the same.
And none of the Aurum Cantus speakers currently made, no matter how expensive, go as low as the SVS CS-Ultra subwoofers. Really, unless one cannot have a subwoofer for some reason, buying a tower speaker is almost always a waste of money with modern systems.
I should point out, the above is assuming you properly adjust the subwoofer and use bass management wisely. If you do not properly set it up, then you will likely hear differences caused by different levels of bass coming from the improperly set up subwoofer. Here is the real downside to a subwoofer: It enables the user to get its level wrong relative to the main speaker, whereas a full range speaker has everything already set up properly relative to each other (this is done in a well designed crossover to match the levels of the woofer and tweeter).
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*This is true of the older version that I have; the new Leisure 2SE supposedly has a better mid-bass driver than the more expensive speaker.
