JC3+ With Kleos SL?

64dB is more than enough for the Kleos SL.......at 0.25mV output you need about 61-62dB of gain for optimal output. I suspect anything more and you risk introducing too much hiss.
Use the KAB Gain calculator for an idea......... 

I wouldn’t go that route, what minimal increases you are likely to gain with the lower output SL, you will almost certainly lose with pushing the phono stage into potential distortion. 
Probably depends on how much gain you enjoy listening at, but I always believe that the lowest output cartridges essentially throw away their gains with the losses at the phono stage. I’m no fan of these ultra low output cartridges for this reason.

I’m with Catcher. With 90%+ of the systems out there 64 dB will be plenty of gain for a .25 mV cartridge. You could drop to .2 mV and likely have no problem for lack of gain unless you’re running a system with a passive preamp or horribly inefficient speakers.

.25 mV would be considered low output, but hardly ultra low output. I'd reserve that for cartridges outputting less than .15 mV. Regardless, it is all about proper gain matching and 64 dB will be plenty, and most likely a bit more than necessary, not less.