I just read the ASR review/measurements of the Parasound Zphono.
It seems it is pretty much as good as it could possibly be except for a slightly higher noise floor than some other competing phono preamps. However, its SINAD suffered because of 75dB SNR. As Amir pointed out, that puts it in the middle of the pack regarding SINAD. However, is SINAD the one and only most important determinant of an RIAA stage's quality?
What about distortion? In that regard, the Zphono is very good. 0.017% THD at an output of 1.1V rms is quite good enough.
In my opinion, the overload behavior and headroom of an RIAA stage are extremely important. If the RIAA stage overloads on every tick and pop, it will exaggerate those noises and 'sound bad'. That would not be good. In this regard, the Zphono looks good too. Amir found the headroom to be better than other budget preamps, and it is not frequency dependent. It didn't approach overload until 44mV rms input at 1kHz (with 47dB gain that would be 8.8V rms out, which is pretty impressive!). That means the THD will be low in all normal usage situations, including hitting most clicks and pops. I rate that as a very good thing.
Finally, there's this question of gain. I like my music sources to all play at a similar enough volume level that I don't have to adjust the volume control when I change sources from LP to digital to TV, etc. The 'standard' max output level for digital sources is 2V rms signal output at 0dBFS (full scale). That's actually quite 'hot' compared to vintage analog sources like FM tuners and older RIAA preamps.
I've found that if I'm using a typical 'hi-fi' (not DJ) moving magnet (MM) cartridge like the Audio Technica AT-VM95ML with about 3.5mV nominal output, it takes about 42 to 45dB of gain to bring the volume level from LP up to about the same level as from either of my two DACs or my CD player (all rated at about 2V rms out at 0dBFS). If I use an RIAA stage with 35dB gain, I find myself turning the volume control up almost a quarter-turn to get LP playback up to the same level as digital source playback. It's a minor concern for many, but I don't like being forced to fiddle with controls all the time.
The Zphono's gain at 1kHz is shown to be 47dB. That is on the high side, but if you're using a cartridge with fairly low output, like a Denon DL110 (2.5mV), a Grado (4mV), or some Audio Technica MM cartridges (3.5mV), that extra gain will be welcome. If you use a DJ cartridge like the Shure M44X then it will probably not work well at all (too much gain).
Will gain of 47dB from your RIAA preamp overload your amplifier's input? Well, that depends on how your system is configured. Do you use a line preamp with gain? What is the input sensitivity of your amplifier? Does your system overload from digital sources now? Remember that after the RIAA stage comes the input selector and volume control (usually a resistive voltage divider). If you need to reduce the signal level from the RIAA stage you turn down the volume control. If the system isn't already prone to overload, I don't see how a 'hotter' signal from the RIAA stage would cause it to overload.
The iFi Zen has 36.5dB of gain at 1kHz, which is a whopping 10dB less than the Zphono. You will hear that difference. You will need to turn the volume control up quite a bit to match levels between LP and digital sources. Will that matter in your system? Even though the iFi Zen has very low power supply noise, its SINAD comes out 8 dB worse than the Zphono's, in part due to its low gain.
Also in the iFi Zen review, you can see that its THD rises and headroom decreases with increasing frequency. I wonder if this means its overload behavior from clicks and pops will be noticeably worse than from other preamps with more headroom up there.
The Cambridge Duo measures best, but its gain is 39dB at 1kHz. That's probably a good compromise but still on the low side for me. It also looks like the Cambridge Duo has good headroom. Amir states that he didn't see THD spike upwards until the preamp was putting out 6V rms. That's quite good for a solid state preamp, although many tube preamps can swing more volts than this (they work at high B+ voltages, so that's their strong suit). However, unless I'm reading Amir's measurements wrong, it looks like the Zphono has significantly higher headroom than the Cambridge Duo. Am I figuring that wrong?