Have you tried wet food for your cats? I used to feed dry, too. No more urinary problems since mine have been on 98% wet food. It's much better to feed wet, particularly for male cats who can block up so quickly. Been there, done that. One male blocked with struvite and a female with oxalate. No UTIs since they have all been on wet food.
Yup, I agree - no dry food for any cat. Read www.catinfo.org
for reasons why, and the connection to common feline diseases
For cats with CRF, there's info out there now that suggest that the best diet is a wet diet, because it helps keep them properly hydrated which is very important in CRF. There's some links to articles at this site: http://felineoutreach.org/EducationDetail.asp?cat=KidneyDisease