It isn't JUST impedance that one has to take into consideration. It has A LOT to do with the current supplied, and the current able to be handled on the circuit board. Therein lay the answer. It's still not really a clean cut answer, but believe it or not, whether or not the line out is "true" or not, has VERY LITTLE to do with whether or not the DSP stage or EQ stage is bypassed. A true line out can be measured by "the numbers". Unfortunately, Iriver has never disclosed the exact info needed to discern the facts.
P.S. You do NOT want the Sony V 600's OR the 900's. V6's ..sure go for it, they're great. I use them with nothing BUT the line out. An amp would make them perform even better. Bottom line without having to provide written technical proof: Using cans with a nice portable amp from the line out = Better performance all around. This has been proven over and over. Ears don't lie. People do.
The 50's are 60 ohms ? That's pretty high for those jammies. The 35's are 32 ohms.