Hancoque, yeah, I agree with that.
Seidhepriest, looks like I won't have to code it. Check this out:

Top one is 5Khz, bottom is 8Khz. Not sure where those large notches are coming from, but it works fine on other frequencies I've tried.
The other weird thing is my DAC all of a sudden developed a ringing at 96Khz today (you can see it on the top graph). I can't get a DC signal out of it, whereas yesterday I could. Weird.
But yeah, apart from those two issues I'm happy with this upsampler. It retains decent square edges below 10Khz and keeps very smooth sine waves above that. The best of both worlds

CPU usage is 50% though >_<
LawnGnome: Grow up.