Hey utdeep, thanks for the response!
The Late 2010 MBA has a Cirrus Logic CS4206A sound card built in. Not much other equipment in it. I'm not sure what soundcard is in the MBP but I would bet that it's equivalent if not better than the MBA.
I'd define significant difference as a very noticeable difference in sound quality. Of course it has to be matched by a not-overly-significant difference in cost but the value of money can be very subjective. For me, a USB DAC <100USD that would make my music sound better would be a great investment.
Speaking of which, I listen to a mix of lossless FLAC and 320kbps MP3. I guess that my home setup would be somewhat mid-range audiophile and my mobile setup is pretty entry level.
I'll probably wait for my M80s to come in and try out the UD100 or Hifidiyme at a local store.
Asus essence stx + Matrix M-stage + AKG Q701:
I agree with most reviewers that the Q701s are mixing 'phones. Excellent clarity and separation. Highs may be a tiny bit shrill. Bass tight but rather underwhelming for pop, R&B and trance. Can be a very enjoyable combination with some minimal EQ adjustments.
Asus essence stx + B&W C5
Powerful, tight bass. Clarity and separation can't match the Q701s though
Late 2010 MBA + B&W C5
Tight bass but not as powerful as with the Asus STX. Similar clarity and separation as with the Asus STX. Looks like these in-ears perform at their best with a good built-in sound card. No point upgrading if I'm just using these.
Asus essence stx + Denon AH-D7100 (borrowed)
Very nice, warm sound with good separation. Hifi rather than reference (like my Q701). Makes everything sound good. Separation doesn't compare with my Q701s. Would love to get these but at 4x the price of my Q701s, I'll just settle