I thought I would also mention that my floors are oak...
I really really appreciate anyone's help. The guys are rushing me to make a decision.

I used Dark Walnut stain (2 coats) on oak for my stair treads and they're pretty darn dark. Sorry for the blurry pic; it's the only one I've got on my laptop.

"Masking" urine stains is virtually impossible. The boards should be replaced. Or the next hot day your home may smell like a cat lady house.

Dark brown floors show dirt and dust way more than medium toned ones do. Be sure you are up for daily Swiffering if you choose a dark floor or you will come to hate it.

Plus, no matter what you do, you need to replace the urine soaked wood. There is no "disguising" that when summer humidity comes around.

Thank you all for your responses. I've already gotten a coat of stain on before seeing them. So, you are saying that even with a coat of stain and 2 coats of poly, you will smell urine on hot days? Yikes, I sure hope not! It was hot during and after the sanding process and I never noticed a urine smell. The stains were several years old and I'm guessing they were pet urine since the previous owners had lots of pets.

Sometimes there can be lingering urine smells from compromised wood and sometimes not. The urine crystals can also compromise the finish in some cases. If you detect no lingering odor on the worst summer days and the finish remains intact for a year, then you probably won't have any of those problems with your floor.

Here's a method for getting a very dark brown with wood dyes.
Base it on "dark brown mahogany" color dye. Add black dye until it is dark enough. Then if necessary add dark red dye drop by drop until it's warm enough.
Casey

If you do not mind really dark, even running to black, use 'liquid nightmare', AKA asphaltum.

Cut it with paint thinner, apply, allow to dry, ad than cover with a solvent based finish.

Make test pieces up.

You are all very kind to reply... thank you so much! Brickeyee... do you have a photo of floors done with liquid nightmare and where do you buy it?

Any place that sells general roofing supplies should have it.

No pics.

It is a 'cut and try' (or in this case dilute and finish).

deesent ... He's talking about using diluted roofing tar as the floor finish.

It's definitely dark, but not a good finish for floors.