Been playing around with settings on the Studio Quality Check Version. It's a good way of opting out of the "keeping up with latest version" race
On the first page of settings, Menu 1 is set as usual for zero sound degradation. I've set Menu 2 for x4 upsampling and enabled sound optimisation.
On the main player page, to keep things manageable I varied only one parameter while keeping every other setting constant. The constants for my purpose were:
mmx+ : plus, mistake
Bug Level : 3
Kitty: Level 3
Jitter: Level 3
Jitter quality: null
sGalaxy: Star Symphony tuning
Phase correction: Auto A
Threads: 4
With the above kept unchanged, I cycled through the x4 settings.
First choice in x4 is BASIC. Within BASIC there are again many flavours. Of these both White x4 and Banana x4 sound quite good though both lacked the fullness of sound heard from other more advanced settings within x4. I think I also heard an ever slightly nagging hardness in both these settings.
After BASIC, there are a couple of Test Sounds to choose from. Of these, Magnet x4 sounds quite nice.
Later I tried Emerald, Sapphire and Snow (no time to try all, but will try to listen the other x4 flavours in future).
As one progresses through the Emerald1 x4, 2, 3, 4, and Sapphire1 x4, 2, 3, 4, and Snow1 x4, 2, 3, 4 one thing becomes clear - the lower numbers consistently sound lighter and airier. As the numbers go up, sound becomes denser. It is is not easy to discern the difference from 1 to 2, or 2 to 3 but one can easily hear the difference between 1 and 4. To my ears, Snow3 x4 sounded very nice, but it wasn't night/day difference from Emerald or Sapphire. They're more similar than different. Other yet untested flavours may tell different stories.
Having arrived at (for now) Snow3 x4 as good sounding, other parameters were then varied, one at a time. I cycled through sGalaxy, Star Symphony and Slash Crystal tunings. There are 3-4 more but I could not go through all yet. Here, I still stick to my earlier choice of Star Symphony tuning.
Finally, for tonight, I varied the Jitter level from nil, then 1, followed by 2 and 3. Level 3 gives me the bass weight I like without overwhelming the rest of the musical spectrum. Here too, the lower numbers consistently sounded lighter. Very high number didn't work for me as the bass weight overwhelmed everything else, eating up resolution in the mids and highs.
I didn't find too much change in the sound on varying the Jitter quality from nil to higher numbers. I also played with Bug Level and Kitty Level but didn't hear much difference when varying either, though higher numbers thickened the sound, and they weren't much to my liking. More listening is required to be sure.
As one can see, the process is tedious and there are no shortcuts. To ease the task, select a track you're very familiar with and play at the most a minute of the track. Otherwise you'll end up spending too much time, and possibly lose track of the subtle differences the parameter changes brought about.
As an acid test after all these faffing and general audio tomfoolery, listen to a few familiar tracks and see if they do sound better. Most likely it will.
Try this at home