I do own a few vintage SPU, and listened some variation, the SPU Gold GE/AE and SPU Synergy impress me most. All SPU produced before 80's are much better than those in latter production.......
With no offense, your BL-51X tonearm is not a good match with the SPU, it a at most a mid mass...
Supex 900 Super was my first MC cartridge in 80''s, After all these years, I still have SD-900Super, SD-909, SD-900E(Wood body), SDX-1100R, SD-700Type IIE, owned and sold are SD-909Mk2, SDX-1000, SDX-3000, SD-801, SD-301, SD-700A.........
The SD-909 is mounted on SAEC WE-506/30 most of the...