I bought the Creative Pebbles Pro to have some safer monitoring option and while playing today I got a noise burst while accidently setting the frequency of the LPF Ugen to 90000 kHz. Can this result in instabilities or did I screw another parameter maybe?
Still, the Pebbles can get quite loud and luckily i didn`t have headphones on.
Will try to reproduce tomorrow at zero volume 
Yes, that will blow up unless you’re running at 192 kHz.
The feedback cycles in infinite impulse response filters are unstable when the frequency gets too close to 0 or to the Nyquist frequency (half the sampling rate). How close seems to depends on rq – larger rq (wider filter), the further away you should stay from these boundaries. In general, cutoff.clip(20, 20000) is safe at 44.1 kHz and above.
hjh