OK, first, Ruffles says to tell you 'we are NOT aged, we are senior, distinguished, kitties." Ruffles is 15.

This is interesting, as it sounds like he is consistently using the tub. Jumping in and out of that must be at least as much work as in and out of the litter box. But maybe there is something making the entry to the box not clear and open? It doesn't have to be anything that changed, just that arthritis and such is making what used to be a quirky jump too hard to do now. That is one possibility.

And I thought a urinary tract infection would mean he is going in a few different places, as the urge came, though I haven't dealt with one. So I bow to others who know more on that topic.

Does the box have a hood? Maybe the smell has built up in the plastic to the point he doesn't much like it in there anymore? I know you said they scoop regularly, but maybe they need to just empty the whole thing, give it a good scrub with some dish soap and a scrubbing brush, and start fresh? Mr. Amber Cat was VERY particular about his box!

If it is a small size box with a hood, and if he is a large boy, maybe he just can't swat low enough to get comfortable in there now? Mr. Amber Cat had some back issues the last 2 years of his life, and I had to remove the hood so he could use it. He didn't switch to the tub to let me know, though, he just started using a corner of the room.

Just some thoughts.