Phonostages with Adjustable Curves, the Concise List

EMT JPA-66

http://emt-international.ch/e/jpa66.php

Why do you need an adjustable RIAA curve ? For which records? 
It's much better to have RCA plug sockets for load resistors (to load or unload the cartridge). 

@chakster this recent review illustrates the value of multiple curves to suit the way specific records were cut (nothing to do with loading)
https://positive-feedback.com/audio-discourse/gold-note-ph-10-phono/

chakster

Why do you need an adjustable RIAA curve ?

I don't think the OP was asking about an adjustable RIAA curve, but about the non-standard curves that predate the RIAA standard, such as the Columbia EQ.

If you play lots of records made before the RIAA curve became the standard, such flexibility is useful, even necessary for historical archiving. Otherwise, it only introduces extra switches, components and solder joints that can degrade SQ. I once rebuilt a Quad 22 preamp, which offered several curves — I never used them so I stripped them out. I ended up with almost a meter of wire, in many little 1- and 2-inch lengths, plus several switches, little caps and resistors, left over on my bench — and a far better sounding preamp.