I have a Musical Fidelity M1DAC and M1HPAP which are both excellent DAC and amp in the 1000 dollar price category according to all the reviews and experiences of people using them, and even match the sound signature of the T1's perfectly, and I still don't hear a "massive" difference between HD650's and T1's. It comes down to a sound signature difference much more than the actual sound quality difference. It's not like taking HD650's off and putting T1's makes me go wow, there is a difference, but nothing major, and there's nothing missing from the HD650's (and therefore HD600's) that would in any way degrade the listening experience, it's just a small step below. Like going from a 50 inch TV to a 42 inch TV, yes, it's slightly smaller at first, but after 5 minutes you get used to it again and you can watch a movie just as well as you did on the 50 inch one, ultimately having the same experience. Some people make you think that upgrading to x thousand dollar headphone amp and DAC will be like going from that 42 inch TV to an iMax 3D movie theater, which is just not true. I'm just being realistic here. I don't really see how people hear massive differences between these headphones, because I never did. I've heard both of them out of some other really respectable amps like the Lehmann BCL or Beyerdynamic A1, and still the T1's sound just like T1's, they don't magically change when used with a slightly better amp. All the changes are really minor and we're really talking about the evolution of the sound rather than revolution. I'm not saying HD650's are as good as T1's, or that T1's don't improve with better gear, but I think it's really important not to over-exaggerate when it comes to gear upgrades, because that really makes inexperienced people spends tons of money on something that's going to only marginally improve their listening experience. That's why I say he should definitely not buy anything without trying it out first, because I know from my experience of owning the same gear as he does, plus much more expensive stuff, that the much more expensive stuff makes nowhere near as much difference as he hopes it will, or as some people suggest it will. Also, I really think people should not suggest someone to change their current setup with something else, if they haven't personally tried the setup that he currently has. Just my opinion. I wish someone had told me what I'm telling him now, might have saved me a couple of grands, instead I believed all the suggestions and hypes, and every single time, without exception, ended up with a sour face after discovering that what I've just payed 1000 dollars for (for example Musical Fidelity M1DAC) is barely distinguishable from something that costs 200 dollars (Asus Essence ST in this case).
To be honest six month ago I would have been one of those who agreed fully with you. But after using both HD650 and T1 with the BH Crack and then embarking on the Cracks well know upgrade path. It didn't take long for the capabilities of the HD650 to reach a point where further noticeable improvement came to a end However the T1's on the other hand came alive with the modifications dynamics resolution timbre tone all improved. Total including the Crack around $600 including some decent tubes. Now there is no comparison going back to the HD650's they feel quiet linear. Details that the HD650 might only just hint at are on the T1 are well defined and will have further details and nuances of tone and timbre in their own right.
With regard to the dac I would expect your actually limited by your amplifiers ability especially if your running something with a wire wound standard pot and electrolytic output capacitors they can act as a bottleneck and it just wont matter how expensive or good the dac is you put in front.