Bass wavelengths are larger than the boundaries of most rooms. This induces standing waves which makes accurate bass reproduction a matter of chance, if left alone.
ADTG points out you never hear people complaining of muddy bass in book shelf sized speakers - they don't tend to offer much (not an absolute, but typically) below 70-80 hz.
80 hz is a wise crossover frequency because, depending on the size of the room, the wavelength reaches a size where the bass signal stops being omni directional and gains forward bias with increasing frequency.
This also means that the bookshelf vs tower debate really just comes down to room size as you will need subwoofers to achieve equal bass quality at all seats, and all frequencies, regardless.
As you already have a tower, the RF7 by itself would probably be fine without a subwoofer for casual listening. You won't hear it all, but you'll certainly here most of the bass. Muddy/boomy bass is usually treated with absorption. If that happens, add some pillows and you're good!
