What is meant by HD6xx ?

[Crankshaft]

What if meant when reference is made to the Sennheiser HD6xx ?

I know of HD600 and HD650.  Is HD6xx a different model or just referring the the 600 series?

 

[Mat]

 

[Mat]

 

[Crankshaft]

 

[Scubadude]

 

[capetownwatches]

 

[croak]