Hi Tranche,
I can only speak to the models I have, I really don't like making comments about things I haven't heard. Going from the the RS5 to the Alnico, is very much a change. You'll see it mentioned over and over here that the Alnico provides a texture to the notes that the RS5 doesn't convey. Really, it just sounds like all of the sudden, there is more information being provided with every note. It's very intoxicating. Also, the Alnico, being in a bigger box with a bigger driver does go lower and naturally sounds like a bigger speaker.
The RS5's strength is that it conveys speed very well. It's strength over the Alnico might be that it's just a little bit more coherent from top to bottom. At times, the Alnico can emphasize a very very slight and narrow peak in the treble. Perhaps this is an artifact of the whizzer cone? It's not there very often, for example, you can listen for hours and never hear it, but then it'll pop up in one song and you'll think...that's not there with the RS5. Is it the actual Alnico driver or is that driver just telling you what's on the recording....that I can't say. Again, you could listen for weeks and never notice it or maybe not...depends on the quality of your source material. Compared to conventional speakers, both are an order of magnitude more coherent.
I'm guessing that adding a sub to the RS5 will not give you the texture that you get with the Alnico, but I'm sure you'll get gains in the usual areas when you add a sub. Honestly, you can't go wrong with either and as I said, if you like what that Kanspea is doing, you'll be VERY happy after you hear the Omegas.