Anyone know why Sennheiser ships the HD558 and the HD598 with 1/4" (6.35mm) headphone jack?
I'm lucky that my sound card and Yamaha receiver have 1/4" jacks.
Do you think that Sennheiser is just trying to make their headphones look more "High End"?

The ED8 ships with a 1/8 and they're $1400.
 
Anyway, they're meant for home use (open headphones, quite large as well), so it makes sense.

That may be the case, but the problem is many potential buyers will be plugging them into 3.5mm jacks(mini stereos, mp3 players, portable amps, etc.).  An option for a standard 3.5mm plug termination would be very much appreciated.

You're not the first one. This is annoying. 
 
A sennh rep said this:
Quote:

The HD 598 was designed for home hi-fi use, hence the original 1/4" termination.

Click to expand...

 

Senn's current HD5xx line all come terminated in 1/4" jacks. But yeah, it would have been nice if terminated them in 1/8"... I mean, my Beyerdynamic DT150 clearly weren't made for mobile use as they're very bulky but I appreciate the 1/8" termination.

My 598's came with a 1/8 adaptor along with the phones which were terminated
with 1/4".
regards,
Hibuck...

Who would wear circumaural open-backed headphones outdoors? Portable amps aren't popular enough with normal consumers for big manufacturers to assume that everyone using the HD5*8 series would have one. Most equipment for home or studio use have 1/4" jacks. I quite like the 1/4" plug.

Good point.  Perhaps I overstated a bit by saying most.  I'll edit my previous post.

"Why do home receivers and amplifiers come with 1/4 jacks, are they just trying to look high end?"
 
 
Large open cans are intended to be used primarily with desktop equipment at home (or in the studio.)  It wouldn't make much sense to put a plug on that's the wrong size for the jack on the equipment it will most commonly be used with.  1/4" is also a bit more robust given abuse over the long haul which is why it's a studio and desktop gear standard. 
 
Of course the HD650's include a 1/4-1/8 adapter cable in the box, which is a handy touch.  And the most utilitarian plug is the approach used by Sony, AKG, Audio Technica etc:  The piggyback plug where it's a 1/8 termination but the 1/4" adapter screws on, making it look and act like a dedicated 1/4 plug as a single unit.  For the pro markets they're designed for, the 1/4 is great for your mixing boards, and the 1/8 is great for your wireless stage packs and the TRS out on studio cameras etc.