Ehoen,
If I tried to describe the difference, I would probably mislead you. I'll try anyway.
It's very odd, but the little Velos sounded punchy and dynamic while the Rythmiks don't sound much of anything at all. Just solid and there. I would have guessed that the Velos (2 x SPLR8) were producing more output at/around 80db, but they were measured for the same output. In fact, both sets of subs were EQ'd to nearly identical on-axis FR at the listening position, but sounded quite different from each other (pointing out the limitations in this measurement approach).
Bottom line: The Rythmiks are abit less "dramatic", but in a relative comparison they are also less obtrusive. Some folks might prefer the Velos. I now use one in a little HT system in my den and the other in my dining room where each serves its respective purpose nicely.
Sorry if that's a bit vague, but it's the best I can muster.
Marty