Hello Arthurp,
Well I agree with Jmcgrogan2 and Elberoth2 that the REF3 will have a brighter high end, only thing is that in my view I find it to be unnaturally spotlit. I haven't had a chance to hear the 52se yet so I can't give you a direct specific comparison of these 2 preamps. I do own the BAT Rex and can tell you I find it to pretty unique, it has the most grain free natural rendering of timbre I've heard.
I'm giving you my opinion of the Rex because the 52se can be upgraded to the Rex. You really should try to at least audition these 2 preamps in systems at dealers, of course comparing them in your own system would be ideal.
Good luck with whatever you decide on.
best,
Tom