Thanks both for your response. You seem not to agree.
Youtuber Tarun (A British Audophile) (see
https://www.youtube.com/watch?v=X562tyZPgRQ&t=8s) pointed out that this might have something to do with the analogue input on the M10, which he thinks is mediocre.
He believes it might be caused by the analogue to digital conversion in the M10. Therefore I thought an external ADC (so not a DAC but other way around) would maybe help.
I would then still make use of the DAC inside the M10, but the input will be digital instead of analogue.
What I was thinking is connecting the phono preamp to an external ADC converter and then connecting this to the digital input of the M10. Do you guys think this is worth looking into?
As this is a hassle with unclear results I would much prefer do something 'simple' as upgrading the preamp/phono stage. Any tips on beter preamps then the Rega fono mini?