There are excellent flatwounds everywhere, but if you want good tension with your flats on a short scale bass, this is IMHO what would work best for you (and what I would do-I have my short scale tuned to D, and use high tension La Bella Hard Rockin Steels, and the tension has a medium feel, not floppy at all, but not too tight):
Just Strings
I have tried both Chromes and Rotosound Jazz for my bass, but they offer only one tension, which is usually too floppy (to me, that is) even in standard E tuning. Manufacturers don't seem to consider the specific needs of a few players, and just go for sales. Note that I LOVE roto flats, but the tension on a short scale is quite the big minus, especially if you are tuning anthing lower then E. Chromes are great too, similar problem. Great strings that aren't being thoughtfully produced for short scales, IMHO.
The only flats I would get for my bass, not only because of the tone but also because of their good tension would be those Deep Talkin flats. I was considering the same a few months back, and although I still have my La Bella roundwounds on, the only option for my bass for D tuning must be the Deep Talkin flats-too bad for Rotosound, for I do like their flats (amazing on a long scale.)