Dunno. I've never used iTunes and never will.
I guess iTunes probably supports EQ/DSP enhancement with M4P files...
If you play an M4P in Winamp, it just uses system waveOut
(ie. Winamp's Output system is totally bypassed).
So the only things which will effect the sound quality are the settings in your sound card cp / windows mixer.
All this aside, I can't understand why people are suckered into paying for limited, poor quality, protected proprietry formats in the first place, especially when there are so many decent alternatives
(including legal/paid for solutions), or just buying the CD :/