M1 = "Shitty preamp"
The M1 has the same preamp circuitry as an SBM-1 which is considered a very respectable preamp.
Again, please elaborate...
All the Sony portables - D7/D8/D100/M1/SBM-1 - have a relatively poor quality mic-in stage. When designing a pro-sumer device of such small size in an affordable package, the manufacturer has to skimp somewhere to keep the cost down. In the Sony portables, they skimped in the mic-in preamp stage.
The D100/M1/SBM-1 mic-in sounds better than the D7/D8 mic-in because they're later generations. Same goes for the line-in and ADC on the D100/M1/SBM-1 v. the D7/D8. But they *all* sound better line-in than mic-in because you're bypassing the relatively poor quality mic-in preamp.
There's nothing wrong with running mic-in, you'll just achieve higher quality recordings if you can run line-in with strong enough levels. But by all means run mics > mic-in if you're on a budget and it's all you can afford, or don't hear the difference between running mic-in and line-in, or are perfectly happy with your mic-in recordings!