Hi Mike,
    You should be able to find the info you need on these websites.
If you have a cat that suffers from any form of FLUTD, it's best if
you can feed them on a wet diet ,e.g. canned.

http://www.supervet.co.uk/cat/cystitis.htm
http://www.fabcats.org/is61.html cystitis
http://www.marvistavet.com/html/body_feline_lower_urinary
http://www.waltham.com/cats/clinical_nutrition/lower_urinary_tract_disease.html

Alison
http://catinfolinks.mysite.wanadoo-members.co.uk/

Mike,
  It will be helpful if you step back just a bit and examine
nutrients, not ingredients. Oxalates, struvites, purines, etc. are all
composed of nutrients, not one of them is composed of an ingredient.
Examining a food based on ingredients is kind analogous to simple
third grade elementary mathematics - simple addition and subtraction.
Taking the next step up and examining a food based upon nutrients is
kind of like advancing to senior high school mathematics - algebra and
trig.
  Your cat has no use for a molecule of chicken/venison/beef/tuna
muscle tissue, what the cat needs is the individual amino acids
contained within that molecule of chicken. If your cat needs trytophan
for instance, it really doesn't matter at all if that trytophan is
sourced from beets or buffalo. It does not matter where the source is,
only that sufficient quantities of bio-available tryptophan is being
ingested.
  While it is true that cats are indeed obligate carnivores, there
are colonies of cats at UC Davis that have been on a purely vegetarian
corn gluten diet for many many years, without any ill effects.
  Plant source versus animal source has nothing whatsoever to do with
the type of urolith a cat may develop. Rather it is the provision of
excess amounts of magnesium, phosphates, and a urine pH that is too
high >6.5 that contributes to struvite development. In calcium
oxalates it is the levels of oxcilic acid and calcium ingested and a
urine pH that is too low <6.2
  Certain breeds of cats are more prone to oxalate formation than
others. Burmese, Persian and Himalayan are all oxalate prone. Older
cats >5 yrs old are more likely to form oxalates than younger cats.
Some manufacturers take this risk status into perspective when they
create foods. Senior cat foods tend to create increased urine pH in
the 6.6-6.8 range, younger adult foods will usually be structured to
create a urine pH between 6.2-6.4. Narrow urine pH values indicate the
manufacturer is spending time and effort to control urine pH levels.
Wide ranges like 6.1-6.9 indicate the manufacturer either can't
regulate the production process well enough to provide a consistent
urine pH, or doesn't care to do so.
  All dry diets are predominantly grain based. That means they
contain more grains than they contain meat proteins. For the purposes
of urolith prevention you should be looking at canned foods. Canned
foods force more water excretion through the urinary tract and thus
dilute the urine. Diluted urine is less likely to form uroliths. Dry
foods, even with added water, tend to force mroe water through the
feces and leave the urine more concentrated.

We use the royal canin so and Hills x/d for preventing oxalate stones.
There is controversey about the royal canin's effectiveness since it
acidifies the urine thereby supposedly preventing both types of stones.
Some internists are suspicious of the claims that the same food can prevent
both types.  Therefore my choice would be the x/d.  I strongly encourage
feeding only the canned food version and discontuing kibble altogether.
Canned food creates a more dilute urine in which stones are less likely to
form.