Finally found the problem.
another bug in the RX-888 driver.
There is a comma too much, as such it ignores -m low without --pga is used.
I did a pull-request on the driver. As without it, this like won't work:
rx888_stream/target/release/rx888_stream -f ./rx888_stream/SDDC_FX3.img -s 60000000 -g 73 -a 0 -m low -r -d -o - > rx888mk2.fifo
Find this part in /src/main.rs
let packet_size = 131072;
let num_transfers = 32;
let gain = match args.gain_mode {
GainMode::High => args.gain | 0x80,
GainMode::Low => args.gain
};
The low line holds a , at the end, once removed it works.
I also noticed this mistake is made many times, with the last setting a , behind it. Never seen code where this is normal.