[Raptor007]

Happened to find this thread while figuring this out. Since ChillToMusic already bumped it this year, figured I'd reply with my solution.

I just got the iFi Zen CAN headphone amp, and intended to add it to my setup while continuing to use a Marantz SR5005 AV receiver to switch inputs, drive my speakers when desired, and otherwise act as DAC for the Zen. Frustratingly, the receiver thwarted my initial attempts to use any of its RCA outputs for this. As my input signals are both optical digital, the Marantz manual revealed why [p.41]:

It is not possible to play the digital audio signals input from the
HDMI, COAXIAL or OPTICAL terminals in ZONE2. Play using analog
connections (in stereo).

The solution I arrived at is to just connect the headphone amp to the AV receiver's headphone jack, with the receiver's headphone output volume somewhere around -36dB. As you suggested, you could do this by adapting the 6.5mm to dual RCA. In my case the ifi Zen CAN has a 3.5mm input, so I just used a short 3.5mm cable with a 6.5mm adapter on the end going into the AV receiver's headphone jack.

This actually turned out perfect for me. It retains the nice feature that plugging/unplugging the headphone amp from the receiver's headphone jack automatically mutes/unmutes the speakers. It also lets me use the AV receiver's bass/treble tone controls for the headphone jack without applying those settings to the speakers. I haven't done A/B testing yet (maybe this weekend) but my initial impression is that the sound is improved in clarity and impact with the amp, and that's not even using its XBass or 3D enhancements.

 

[Stevko]

is it worth it?
my rxv460 HP output sounds very bad.

 

[Raptor007]

is it worth it?
my rx440 HP output sounds very bad.

I don't regret the purchase, but I can't say if it's the best bang for your buck. I found the difference overall subtle in my setup, though I can find a few songs where it is more obvious. I've been considering planar headphones, but decided to try an amp first to see if that's all my old headphones needed to achieve perfection... and I'm still considering those planars (mostly for soundstage at this point).

For my headphones and ears, I set the Marantz tone control at Bass+2dB Treble-4dB. Without the amp I might crank the volume as high as -21dB for songs with more dynamic range. What I think I hear when doing that straight from the headphone jack is a mild muddling of crescendos; I can't complain about any individual sound, but when a bunch of sounds hit at once they seem momentarily less distinct. With the headphone amp, maintaining separation seems improved. There are also a few specific places in some songs where bass seems more sharply defined. But for some songs I couldn't tell the difference; the Marantz output is really not bad. This is just from a quick non-scientific A/B just now where I had to replug things and mess with the volume knobs.

This is my first headphone amp so I couldn't tell you if the iFi Zen CAN at $189 is any more satisfying than a $99 Magni 3 or something even cheaper. I like that it has the XBass and 3D gimmicks done entirely in analog, although mostly I'm using it without those as I already dialed in the tone control on the Marantz. Theoretically I also like that it has some Class A circuitry for more immediate response and a balanced output to drive the fancy planars I've been eying, but I have no idea if I'd actually hear the difference between this amp and one lacking those features.