As I think has been previously mentioned, the WM-DC2, D6 and D6C all have amorphous heads, but the later EX amorphous heads have greater frequency extension (and sound better generally too, at least to my ears).
As for the D6Cs, I've had a number, and while they've been good, they're not as good as any half-decent full-size deck. The frequency response drops like a brick above 15kHz, and the speed stability, while very good for a portable of its size, is really not difficult to better with a full-size deck. From what I've read, they're incapable of biasing metals properly too, though that's not something I checked on mine (went for chromes instead).
I loved the sound of my couple of DC2s, but the DD9 was clearly better in a number of areas.
Had one of the Marantz portables, but never spent any proper time sorting it. The transport was quite clever though - had twin, contra-rotating flywheels. The Disc Drive of the Sony Pros is good too, though plug them into a wow/flutter meter, move them around, and you'll see how the movement hits the wow and flutter figures pretty clearly.