Now, to really throw you guys a curveball, when I said 'permanent' I was including creatures in my mind.
I could change the text to explicitly include creatures, or just leave it as is if you think adding creatures would make it too broken.
Actually, I think this card is more dangerous with its permanent interactions than creature interactions.
In many cases, the effect of tweaking a number is somewhat akin to creating a new creature. (e.g. you could get +1 poison per turn by buffing an existing parasite or simply creating a new one.)
The ability may become a little more vicious (since you will impact their efficiency), but you can still get somewhat of the same extra power by simply copying the creature (fractal, mitosis, PU etc.) This was taken into account during their creation, so it should be a little harder to break the balance.
Permanents, on the other hand, are limited to 6 per deck with a hard limit (no way to copy them). Moreover, shields (and to a lesser extent weapons) are expected to only have one in play at a time. So this is the most likely place messing with numbers would break something.
If balancing things gets too tough, you may consider limiting this to just creatures instead of just permanents.
