MM or MI Cartridge?

I went through that phase just recently. I have owned Grados and Clearaudios, but the best so far overall was the Soundsmith Voice. It punches way above it's price point with exceptional tracking and remarkable neutrality. I gave it to a friend for his birthday and he greatly prefers it to his old Spectral (really a Lyra) cartridge. My current phono stage will not handle a high output cartridge.

I remain an mc cartridge fan and user because no mm or mi type I have tried can equal the "liveliness" and detail retrieval of a good mc cartridge. Even the lowly Denon 103 is capable of astonishing sound quality! Why don't you consider an upper-tier mc cartridge like a Miyajima Madake? After all, that Ortofon SPU is rather long-in-the-tooth for an mc design!

Nagaoka MP500 is about as good as it gets IMHO, but it’s subjective and very system dependent. It does a lot of things extremely well, and doesn't have any bad habits.  Technically it’s moving permalloy, but I wouldn’t let semantics be the reason to stop you from trying it. 😎

Soundsmith makes a strain-gauge cartridge. This type does not use an RIAA network/phono stage. It does the equalization mechanically. It can drive a line stage - eliminating the phono stage. I listened to the Panasonic strain gauge at a friend's house. It fed the line stage of a Marantz 7 tube preamp into a pair of Futterman H3aa OTL amps into Quad 57's. I was impressed! Soundsmith claims their strain gauge cartridge is a superior improvement over the Panasonic.

Jason, kudos to your friend with the Marantz-Futterman-Quad system. That’s a vintage combination to rival many modern high end systems except it needs a subwoofer. Back in the early 70s, I owned a similar system save for using KLH9s, first one pair and eventually two pairs, in lieu of the Quads.