So I received my X5 this morning and have been listening for a few hours today. It's definitely an improvement over the WiiM phono stage, purely from a noise / hiss perspective. When I first bought the Ultra I had a nightmare trying to eliminate electrical interference from its overly sensitive phono stage, but eventually was able to track it down to an Aiyima power supply that it didn't like. Since eliminating the Aiyima I've been pleased with the performance of the Ultra's phono stage, but it still does have some hiss at mid to high volume, which is noticeable with tracks where there are quiet moments or pauses.
The Fosi X5 is totally silent, with no audible noise / hiss all the way up to the max volume. It's well put together and a big step up in quality from their X2 tube phono preamp that I also own. I find the compact and understated design of the X5 quite attractive, although the orange led power light is a little too bright for my liking (very minor issue). My setup is Project Debut Carbon turntable with Ortofon 2M Red cartridge. The X5 is connected via RCA to the Ultra, then RCA out from the Ultra to my cheapo Musical Fidelity A1 clone, and finally on to the Wharfedale Diamond 9.1 bookshelf speakers. I'm using the lowest gain setting of 38dB as per recommendation in the X5's instruction manual, although I also had no audible noise when using 48dB setting.
It's quite difficult to be certain about differences in sound quality / signature because I can't easily switch back and forth to compare the Ultra to the X5, but I did feel like I noticed one clear difference. I'm particularly sensitive to trebles and found myself having to turn down the X5 due to harshness in that region at higher volumes, which is something I never had to do on the Ultra. Other possible perceived differences were that the vocals / mids on the X5 seemed a little quieter / further away, and the bass seemed a little louder / fuller but these could also just be my imagination.
Overall, the Fosi X5 is definitely a big improvement over the Ultra's phono stage from a noise perspective, and it's well built and looks good on the shelf. Thanks to WiiM it'll no hassle to EQ out the treble harshness to get to my own taste, so I'll definitely be keeping it.