New Cartridge or New Preamp

Built in phonostages are a design solution for convenience. Only a SOTA unit will have one that may compare with a circuit of an outboard.

Unless you're comparing a SOTA piece, it's safe to say a built in will not pack as much engineering and components needed for a proper  built in phonostage.

You don't think a circuit comparable to this is inside the box of your preamp?

One of these can be had for under $2K used. Manley Chinook

Rogue Ares under $1500 used

Your ears will be pleasantly surprised. Every proper table setup is driven by a separate phonostage.

You likely heard the difference between going integrated amp to stereo, then to monoblocks?

Really, everything is important. For my system the greatest improvement was obtained when I cleaned up my power. Second, a used phono preamplifier. I say this because in my system I went from an integrated with built in preamplifier to a $2400 new preamplifier and ended with a fairly high end preamplifier purchased used that made a major improvement in my system. My turntable is much like yours and I also greatly upgraded my cartridge and this did make a difference, but I don’t think it would have heard as much difference with a lesser phono stage.

The other advantage of going with a separate phonostage is that it opens up the world of LOMC cartridges. Unless you got a separate SUT you likely don’t have the gain to run these with your current setup. If the sonic goal is greater detail then this would be a reason to move in that direction.

Thank you all for your responses so far.

@tablejockey thanks for your detailed response.

I’ll definitely consider adding a better phono stage to my setup. 

Being just a little obsessed with moving iron pickups, I have acquired a Soundsmith Sussurro MkII along the way. It's a couple of steps higher up their food chain than the Zephyr, but even so it has been a disappointment. It seems bland and unexciting, a complaint I can also level against the Grado Statement 3. Playing with resistive loading (using an MC input) helps, but doesn't save the situation. Frankly, the Nagaoka MP-500 walks all over the pair of them.

I'm in the process of adding two extra tonearms to my two turntables. So far, the London Reference, Ortofon Kontrapunkt C, and Nagaoka MP-500 have earned their seats. Rather than use the Sussurro or Statement, I'm thinking the last spot should go to a mono cartridge (or, Gawd 'elp me, one of those two with Y-connectors to make a bridged mono cart!)