First impressions

[efung]

It's interesting.... so today I listened to a few albums, Daft Punk, Radiohead.... LCD Soundsystem, Zeppelin, Goldffrapp, Beatles.... eyes closed... listening intently and ... for me... I found that with Sox turned on there was a lack of low end compared with off. I also found better stereo separation with it off..... Sox on seemed to deaden the sound a bit for me. Without was that bit livelier.... 

So I'm mostly listening to acoustic classical. More complex sound with direct sound and hall reverberation. As philrandal said that the soundstage is better defined. Instrument placement is better. I actually think the "livelier" sound you are describing is distortion. The lack of low end is something I have not experienced.

I don't know how you are powering the RPI/Boss, but for me using an Aukey USB C 20000mAh battery made world of difference, especially in the bass. Highly recommend a battery instead of a power supply. Connected it to the Boss with the jumper on, so it powers the RPI and the Boss. You can play it for over 12 hours. The sonic results are much better than a 5V 20A lab grade Acopian linear power supply that I was using.  I also tried two batteries, one to the RPI and the other to the Boss with the jumper off. Never could get it to work. It's possible that the voltages of the two batteries were very different. The correct method is to power the Boss and then the RPI in this setup. On retries it eventually broke the SD card. Reflashing is easy, but I have a huge music library of over 1TB, so it takes a long time to rescan.

I own a BOSS card. I had experienced the RPi's system restarted periodically when the 5V power was not injected from the RPi4's USB C connector. I guess the problem is due to the 5V power being brought to the bottom side of the PCB with a few small VIA holes since the surface mounted 40-pin dual row pin header is mounted on the back side of the board. This is the reason I avoid to use the SMT 40-pin header in my design. If I chose to use SMT pin header, I would use bigger diameter VIA holes and put more VIAs on the board. It is the misleading information that linear power supply should be prefer for audio design. Linear power supply always carries trace of 50/60Hz from the power line which is difficult to get ride of. Switching power supply has its own advantage that the high switching frequency is easy to filter. If you look at the inside picture of the SMSL Sanskrit you have (Amazon shows the picture), you can find the small dual rail DC-DC converter under the subtitle "Built-in high-performance power". The filter is LC filter. The silver color capacitors are aluminum hybrid polymer type. Some audiophiles use the brand name OS-CON to represent it. This type of electrolytic cap has very low ESR. They are not cheap for its small capacitance. The DC-DC converter is also used to create an isolated analog ground plane. The noisy digital ground plane is due to the fast parallel bus DRAM access. So, if the DC-DC converter's dual rail output has its own ground reference, the noise on the dual analog voltages will not be affected by noisy digital ground plane.