If they are passing a lot, then as you have done, you need to review their diet. It would also be wise to have a vet check carried out just to make sure all is well.
Do be aware that they do still need calcium and a diet too low in calcium can be as bad as one too high - its about finding a balance - so cutting out everything wont be good either.
They need unlimited hay.
They also still need some veggies daily - just ensure high calcium veggies (kale, parsley, spinach) are kept to one small amount once per week.
Keeping pellets limited to one tablespoon per pig per day or even (if you are worried) still feeding pellets but only giving them a few times a week will ensure they still get the essential balance of nutrients that they need while following a low calcium diet. If you cut out pellets entirely, you need to make sure you replace it with forages and other food items to make sure they are still getting the right nutrition.
It will take several weeks before any changes to their diet to have any effect on their system so you will still see powdery pees for a while.
Long Term Balanced General And Special Needs Guinea Pig Diets