On paper I think they are different, especially for the CD6002 since it is actually a little newer than the x001 players. You can find the specification on their website.
I compared the CD5001, CD6002 and SA7001. Of course they sound different and there are no way to compare their headphone section alone. But IMO, they all share the Marantz lively and bright sound signature.
All three drive my notoriously hard to drive K501 very well. The CD6002 need a tiny bit more travel on the volume knob than the other two, but plenty to drive the most power hungry headphone (may be save for the K1000). I haven't heard the SA8001, but I'd be surprised if it is very different from the SA7001.