So after more than a year, coming back with many comments;
I have purchased the P1 and also life gave me a chance to keep Accuphase C-47. Bottom line P1 is better than C-47. How much? Is it worth x3 price? Depends on personal preferences. P1 is more, neutral, more detailed and has way lower noise level. Once set-up to best with test record and ear (I personally prefer, at least 1, sometimes 2 +5 dB low on P1 and more volume on my pre-amplifier) P1 is as good as it gets. If You have a C-47 but a P1 is out of Your budget are You missing too much? Personally no, keep listening and be happy with C-47, it is a wonderful phono pre especially for its price. I also have L1 and X1. Since I would have and keep them, didn't do A/B/C tests (P1/L1/X1 one or both connected etc. variations) but, I had to separate P1 and L1 so kept X1 with L1. I can't say with and without X1, the difference is day or night, I'm not missing the X1, but that's me. Now I also have C1.2 (dac) and even when my two systems merge, I'm planning to use my X1 on C1.2 and L1 and not buy another X1 for my P1 but use it with its internal power supply.
As for
@jsec 's issue above along with
@Bonesy Jonesy 's; I have had similar problem but not with my P1 and my Kuzma 4pt 11 and ClearAudio Goldfinger on it. Although every case is its own; what I realized on my system is; there is a noise source (in my case it was an LED lamp with wireless phone charging base unit too close to my analog layout) and a weak link prone to RF noise (in my case it was my step-up-transformer) which in Your cases may be cartridge body itself and / or any cable on the way). I've had no issue with my Kuzma -> P1 connected on my MC1 input. Even after I got rid of the noisy lamp, my SUT still picks up some RF noise (environment noise, wireless, phone, etc.). Someday, I will built a simple Faraday cage for it and try again. You can always try a basic Faraday cage Yourself (although hard to apply on cartridge): just tear simple kitchen aluminum foil and cover to a closed box / ball / tube to the best You can around the device and have a go. As I said, although not the perfect cage, it shall cut noise enough (i.e. 20 - 30 dB instead of a perfect 50 - 60 dB) to eliminate most noise. For cartridge You can make a ball of aluminum leaving the bottom open and put it on the cartridge even without using (i.e. playing music) and caring for the VTF if You use. In these cases of phono noise, grounding naturally decreases the hum / buzz noise but in my case no matter what combination of grounding used (phono, pre, even grounding to the radiator etc.) didn't kill the noise completely. In P1, the signal ground and chassis ground are separate. In general, in a regular phono pre they are connected (i.e. one). So connecting the phono ground cable to one or the other grounding post or even more, connecting the two ground posts with another cable should yield no technical danger / harm but may decrease, increase or not change at all the noise.
In most phono cables, if it is built as a phono cable, the left common cable is also a grounding cable and additionally there is a grounding cable to connect to the grounding post of the phono pre, or an additional grounding cable to connect the phono pre grounding post to pre-amplifier grounding post. In most XLR phono cables, due to the nature of the process, there are three grounding cables, one for left channel, one for right channel and one for general coming out of Your cartridge. Would this yield a better grounding? Yes definitely. Would this solve Your case? Most probably not. If the noise is generated by Your cartridge or arm cables, this would decrease the noise, not solve completely. You shall eliminate the noise source, or if not able to eliminate, find a solution (as a Faraday cage etc.).