The 8Cs go down to 30 Hz ±1 dB so you'll still need a subwoofer if you want access to the very low end. The following are Thomann prices: 2 x 8351B + 1 x 7370A = 10,719 € (though you can get them cheaper if you shop around and haggle, especially smaller dealers). The DD 8C Studio (pair) costs €9.710 on the Dutch&Dutch store. I suspect if you add a third-party sub to the 8Cs (AFAIK they don't do a subwoofer yet) the combined cost is similar to that of said Genelec system, so it's hard to claim one is better than the other. One way in which Genelec has the upper hand is subwoofer integration, which is a piece of cake if you use GLM (BTW they usually throw this in for free if you buy a pair of 8351Bs). If DD offered a subwoofer, the combined cost would probably be a lot higher than the cost of the equivalent Gennie system.
I know many people around here would like sound to come from a single point source and I used to be sceptical of subwoofers myself for similar reasons until I actually tried a 2.1 Genelec system two weeks ago and now I'm sold. The subwoofer takes work off the monitors and makes the system much easier to integrate into a room as you can put the subwoofer(s) where it sounds best and the monitors where they sound best, which are often two different locations.
I had been using a Subpac till recent and, while it's a perfectly valid tool for mixing and getting right the low end, the perception of bass is different than that of a subwoofer. It's not bad or unpleasant, just different, and you can tell very clearly if you toggle between the two. In other words, either a subwoofer or a Subpac can lead to a good mix, but you are tackling the problem from two different angles. I think the more tools I have to monitor bass the better, hence my desire to incorporate a subwoofer into my system, which in no way replaces the Subpac, and vice versa.