Re: Esp ec401 need advice on replacing the jb
C8, Custom RUOA5, and '59/C Hybrid could all work, depending on how hot you like it. I also wonder about a '59/C UOA5.
C8 has this great, open roar with a punchy bass. Good for almost anything, unless you are obsessed with vintage-correct signal chains.
Custom RUOA5 is somewhere between the strengths of the Custom 5 and the Custom Custom. Warm, but not as syrupy as the A2, with deeper bass. Doesn't have the often excessive mid-scoop of the Custom 5, though the mids are a bit less prominent than the Custom Custom.
'59/Custom Hybrid is just a great blues to rock pickup.
UOA5 in a '59/C makes it a bit more mid-focused, but it still has the broader frequency response from the different coils.
Half of them are a magnet swap away from the rest, so I'd pick the Custom 8 or the '59/Custom Hybrid to try, and go from there. All of them have deeper bass and sound more like the '59 than the JB. But varying degrees of hotter.