[MLee]

I have a pair of Audio Technica's W3000ANV which are rated at 40 ohms impedance.  I've listened to them paired with an Objective 2, a Schitt Lyr, Burson 160, and a Bryston BHA-1.  The two DACs I've used are the ODAC and the HRT iStreamer.  Source is all 256kbps iTunes music.  I've read about output impedance affecting the quality on low impedance headphones like mine.  Some say it makes a difference, that you should stay in the 1/8 to 1/10 range, while others say is is just a recommendation.  The amps I listed range from less than 1 ohm to 10 ohms output impedance but I've never heard anything less than pretty decent quality from all of them.  I also have a Yamaha RX-V1300 that I bought in 2004.  It's nothing fancy, just an upper mid range AV receiver (For it's time) rated at 100watts per channel, discrete amplification, etc.  Who knows what the output impedanceon the headphone jack is.  When I listen to it via the headphone jack the sound is as good if not better than any of the other amps.  I don't know if I could pick it out from the others in a blind test but to my ears there really is nothing lacking.  My question is this a matter of my uneducated ears or does this amp just happen to be a great headphone amp too?  Headphones actually sound better from it than the Marantz SR7005 I replaced it with in my home theater setup.  What is the biggest difference I should be able to hear between a dedicated amp for headphones and an integrated receiver?  Is the quality of my music or the DACs a weak link so it limits the ability to tell a good amp from a great one?

 

[PleasantSounds]

At 102dB/mW your headphones are efficient enough to produce decent sound, even if the source is a bit less than perfect - that may explain why you don't notice any significant issues with the higher impedance amps.
Regarding how it would manifest itself: I'd be looking for a bit "thinner" sound - i.e. expect the bass to sound weaker and less articulated. The reason is that most of the power goes into generating the lower frequency sounds, and with higher impedance the amp may not be able to output enough current. You may also notice increase in distortions accompanying loud low frequency sounds, but that depends on the amp design (class A should be quite resilient to that).
Most AV receivers are actually pretty good at driving headphones, so I'm not surprised at all that your Yamaha is doing OK in that respect.