First kudos for our friends at audiophonics for the Sabre DAC.
Regarding your findings.
Sabre 9018 is a very good IC. The only problem is drivers are not open sourced. From what we know they will never be.
Now Kali was designed to improve the clocks of RPI and I think it does a great job . No matter what DAC board you will get in the future including Sabre (slave DAC) Kali will always help.
Piano 2.1 was designed to take full advantage of the DSP capabilities (xover). I think that Dan from DietPi is implementing it right now. Please retest it then.
Meanwhile , we are launching a few more HATs.
First the BOSS , a master DAC with PCM 5122 (clocks on board). They should be ready for independent testing in 2 weeks. Best bang for the buck.
Second the galvanic isolator HAT. It will work with both master and slave DACs. Needs a extra power supply (we recommend battery )
Third , relay attenuator . It will work as stand alone and for our DACs exclusively . Attenuate volume in hardware using Susumu low noise resistors.
Forth and maybe the most important for all of you. Hiend DAC made by allo. An R2R Dac with 0.01% resistors and reclocking circuit. At about 150$ it will be a game changer. Mark my words. Please be patient on it since we want to make it right .
Oh yeah, we have a 5 project. I call it the Digiless. A recloaked Spidif output with galvanic isolation that will have 4ps (max) measured at output. This again will be a game changer for transporting (a la microrendu)
Enjoy the music . Please post your replies on the allo thread and keep the discussion here for Audiophonics Sabre.