Audio Research Ref-1 vs sp10 mkII preamp

Curious if anyone has an opinion between the arc ref-1 and the sp10 mkII - sonically, that is. assume the sp10 has new tubes, upgraded caps, etc. how does its line stage compare with the much newer ref-1? i've seen highly modified sp10s for around the price of a used ref-1. i'm interested in buying one or the other. any help?
thanks in advance.