A-51 is not the same as A-717. A-51 is the same as A-777 and A-878, and is a 120V US version. A-777 was also produced in SD version for US military, with all voltages available. A-878 is 220/240V only version.
A-51 and A-717 have somewhat similar power amp section with two pairs of the same power transistors for each side meaning they produce about the same power. The main difference is in bias/non-switching circuit implementation. A-717 has Pioneer Non-Switching type III topology based on PA0016 chip found in all high end Pioneer amps of that era, A-51 doesn't have it, and resembles lower end A-400 series amps on that part. Among other things, A-717 has a better phono stage. There are more differences, all playing to A-717 advantage.
I would say A-51 is small a notch below A-717 IMHO.
By the way, there are no Elite turntables to my knowledge. The most fine PL-90 carried the Reference badge. Which understandably did not make her worse in any way.