The 507 has very low mass in the vertical plane. If memory serves, the SPU is a high mass cartridge with low compliance. So there might be a mismatch potential in that mating. I usually see SPUs mounted on 12-inch Ortofon, Ikeda, or Schick tonearms, all of which would be high mass. Also, isn't there something "special" about the mounting of an SPU type that might make it incompatible with a 507.
That said, I have two 505s, have used them with a wide variety of MM and MC cartridges with success. This includes an Ortofon MC7500. But the MC7500 is a very different beast from an SPU.
That said, I have two 505s, have used them with a wide variety of MM and MC cartridges with success. This includes an Ortofon MC7500. But the MC7500 is a very different beast from an SPU.