I don't know how much a water filtration would help with the IR pills, but for XR pills (which have those annoying beads) it will surely remove the time release mechanism.
What I'd do:
1. Crush pills into dust
2. Dissolve in a very small amount of hot water. The hot water will dissolve the binders and hold them in suspension (even if they are not water-soluble).
3. Wait for the solution to cool to room temp.
4. Cool the solution in the fridge. Get it as cold as possible, without freezing it. The amps will still be soluble at low temps, but the other crap will be minimally soluble.
5. Filter solution through a MILLIPORE filter. Discard the filters and the junk they catch. Only keep the filtered solution.
6. Evaporate the water away and you'll be left with (slightly) purer amphetamines.
...But the whole "evaporation" process can be a bitch if you don't have the proper tools. This is why it's better to wash away the solubles than to keep them. The only thing that will help here is PURE alcohol. And the alcohol method is much simpler:
1. Crush pills into dust.
2. Dissolve in pure isopropanol (the amphetamine salts are insoluble in iso and will not actually dissolve).
3. Filter through millipore filter.
4. Keep what the filters catch, and discard the filtered solution.