I've run a .5 mV cart through my ZP3 and followed by the CSP3 with success.  (I was waiting for my SUT to arrive.)  The 2.5 mV will certainly work but ZP3 output will be lower accordingly, so input into your SE84UFO will be below optimum.  I think you'll be well below the 1.5 V input needed to fully drive your amp.  If you currently have plenty of headroom, you might still be okay.  However, a gain stage, post ZP3, may be needed.

Can you replace your stylus?  Or are you itching for a new cart?

See my profile below for which cartridge I use with my ZP3 and no SUT

So I ended up getting the SS zephyr mk iii es and it’s a fantastic cartridge... incredibly resolved, great dynamics, and pairs perfectly with the VPI arm. Plenty of output for the ZP3 and still plays very loud through the SE84UFO and 100db efficient speakers. Have not needed to go past 12 on the amp to get very loud output. This cartridge is only 2.4mv output, but almost seems louder than my previous 4mv output cart. This is probably because it is significantly more dynamic, but I usually find myself playing at 9:30 vs 9 previously on normal listening.

It does bring up another issue of gain vs volume. Like Archie said, I’m far below the 1.5V optimal input voltage for the SE84UFO going directly from the ZP3. There is definitely a sense of “heft” missing from the sound, but this was also missing from my old 4mv cartridge. Plenty of volume without hitting the ZP3 noise floor, just not enough body. My Schiit Modi DAC has an output of 2V and doesn’t sound as good as the vinyl rig, but does have a sense of scale to the sound that vinyl is missing, even at same volume levels. If you’ve ever recorded an instrument or vocals, boosting the gain will drastically change the character of the sound and make it “bigger” at same volume levels. Same situation here. Even if using a very HO MM cartridge like the Ortofon 2M series, im pretty sure you’re still below the 1.5V full output for the SE84UFO. I know this is discussed in other forums, but I think it’s worth mentioning this is more of an issue with the amp rather than the ZP3 as pretty much all MM phono stages have similar gain to the ZP3.

This has lead me to search for a Zstage or possibly a CSP3, however since the schiit is already above the optimal voltage, I really only need the single source voltage gain. If anyone has had a similar experience, I’m interested in what options there are to resolve this at a low cost

I’m sure the CSP3 would make both vinyl and the DAC sound fantastic and give a ton of body to the sound. I’ll probably own one of these eventually but the Zstage (buying used if possible) does seem like the best low(er) cost option. Also the single tube is nice for not getting into a rabbit hole with tube rolling.

Is it best to stick with Decware on something as simple as a single source gain stage or are there other companies/components out there that won’t jeopardize the great tube soundstage of the ZP3? I haven’t really seen much in my search that does something similar to the Zstage with the same simplicity. I want to avoid getting some crazy multiple input preamp because when I have a need for that, I most likely will purchase a CSP3.

The ZSTAGE with its variable gain might be good (seems like a CSP3 light?) although I have no experience with one.  The CSP3 is certainly not the low cost option but I'd keep mine over my ZROCK2, which I took out of the chain a year ago and have no plans on putting back.  However, I've never tried the ZROCK2 without the CSP3.  In my current set up the ZROCK2 doesn't offer anything I like.  Remember, it shifts tonal balance, which isn't always wanted.

The ZROCK2 seems to just add gain in an increment of 3dB, I think.  If you want to "ride the gain," you need a ZSTAGE or like.

Coming back to this thread to share my experience in hopes it helps someone else out there.

I initially had the Soundsmith Zephyr Mk iii es running through my ZP3 directly into the zen and then to some 95db Omega Alnico towers. The sound overall lacked body and punch when I listened to analog. Also sounded rolled off and lack energy most of the time. Digital sounded great, which left me confused. This was not really a volume issue, but rather the character of the sound.

Since I didn’t want to give up the zen/omega pairing for a higher efficiency speaker that might play better with the lower signal voltage,  I purchased a CSP3 to boost the gain after the ZP3. This added a lot to my set up and helped the issue of gain to some degree. The CSP3 really did take the zen to a new level, but still something was still missing from my analog set up. I ended up selling the CSP3 and buying a 300B integrated in hopes this would resolve the issue, which it didn’t, even though the amp is excellent.

At this point I realized the issue wasn’t so much that I needed additional gain after the ZP3, but that the signal going into the ZP3 was much too low. Switching back and forth between the $1500 SS (2.4mv) and $400 Nagaoka (4mv output), the Nagaoka had significantly more energy, body, punch. The SS was unquestionably the better cartridge (higher resolution, better dynamics, wider soundstage, much more transparent, etc.) but something was certainly missing.

Fast forward a few months, I was finally able to sell my Soundsmith Zephyr and replaced it with a Hana ML and Cinemag SUT. The signal going into the ZP3 is now ~7mv! I can’t express how much better it sounds and how I should have done this instead of buying the HO Soundsmith. More energy, more air, larger soundstage, super punchy. More of that liquid analog sound.

This isn’t meant to rag on the Soundsmith at all. The zephyr was probably slightly higher resolution and more timbrely natural, but I prefer the Hana in every other sense as it plays much better with the ZP3. I believe this is largely due to the higher voltage coming out of the SUT, not necessarily because the Hana is a better cartridge. This is definitely the best analog sound I’ve ever had.

Obviously this is just my personal experience and many others here successfully use HO SS carts (or other HOMC or HOMI) around 2mv output with the ZP3. I personally would only recommend only 4mv or higher with the ZP3, especially if you’re a fan of low powered amps that need all the gain they can get. Again, more of an issue of gain than volume.

Steve recommends at least 4mv with the ZP3, which I agree with and I should’ve listened to before I purchased the Soundsmith. What threw me off was that so many people raved about their combination of HOMI or HOMC with the ZP3, which mostly have a output ranges of 2-3mv. I was also against spending additional money on a SUT, rather than having all of that money go toward a cartridge. I only plan on using a SUT and LOMC from here out.

I think your clearaudio cartridge will work fine. The Soundsmith still sounded good but it didn’t have quite enough energy. Your cartridge has over 50% more output than the Zephyr, so I think it would be okay.

My only room treatments are natural to my house - carpeting, a few art pieces that reduce reflections, plants, etc. my issue with the sound wasn’t really related to room treatment as I was satisfied with the sound on digital and now it sounds excellent with the Hana ML and SUT.