How much has DAC technology advanced in the last decade or more?
A lot of significant sonic improvements over the last 8yrs especially in the ~3-8k price range.
Is my Schiit Modi 3+ better than the Musical Fidelity M1 DAC?
Not familiar with this DAC’s performance
And secondly: how much can be 'gained' by spending 3-4x times the value of those 2?
It is much harder to sonically differentiate better DACs below 2k as better parts and engineering, in high-end limited niche market, drive up costs. Whether it’s worth the sonic change/improvement depends on the transparency of the audio chain and the OP’s subjective preferences.