I’m listening to "3 tube" Cary monobocks as I type. I run WE 300Bs and they are stunning. The WE tube was what Cary originally supplied in the amps. Your concern about XLS fitment is well taken. I had a pair of KR 300BXLS and they would not physically fit into the chassis; the openings are too small.
The KR 300B XLS was offered by Cary as a separate model monoblock in the day, not as an optional tube. I suspect there is a reason and I strongly suggest you check with Steve at Cary before you run XLS tubes based on other’s opinions. As my amps are auto bias, I am more than happy to run Western Electrics since they were OEM.
As far as reliability, these are 1999 vintage, had the original tubes until last year and have had no service. Eventually caps will dry out, but I suspect alleged reliability issues may come from people suggesting the use of tubes Cary did not recommend. Equipment can be fussy about which 300B tubes are used. I have a preamp whose maker told me to use Electro Harmonix as supplied, no exceptions and no warranty otherwise. Another integrated of mine with auto bias tried to red plate a set of WE 300Bs, but is perfectly happy with KR tubes. My point is no one cares about the price of a signal tube, but if you are paying $750 per tube, you may want to ask the people who designed, built and service your equipment rather than people who tell you their tube will work "just fine."