I have a Benchmark DAC and recently bought a $150 Lite DAC Ah to use in a friend's system. While I had both at my house I did a fair bit of swapping back and forth because I was curious, of course, about whether I could hear differences between the $1,000 Benchmark and the $150 DAC Ah.
I could, but it was very dependent on the rest of the system, in particular the speakers. With a pair of $400 Usher S-520's the differences were pretty subtle, much like what you're describing. I could hear a difference but not a difference that would justify the price difference to most people.
With a pair of DeVore Gibbons, which cost seven times as much as the Ushers, the differences were much more obvious, enough that the higher cost of the Benchmark makes sense, to me.
It may just be that a DAC that costs as much as the speakers you have doesn't make sense to your ears in terms of bang for the buck. Factor in the high value of the 640C and the Benchmark simply may be somewhere north of the point of diminishing returns for you. That's a good thing as far as your wallet is concerned, enjoy the 640C.
I could, but it was very dependent on the rest of the system, in particular the speakers. With a pair of $400 Usher S-520's the differences were pretty subtle, much like what you're describing. I could hear a difference but not a difference that would justify the price difference to most people.
With a pair of DeVore Gibbons, which cost seven times as much as the Ushers, the differences were much more obvious, enough that the higher cost of the Benchmark makes sense, to me.
It may just be that a DAC that costs as much as the speakers you have doesn't make sense to your ears in terms of bang for the buck. Factor in the high value of the 640C and the Benchmark simply may be somewhere north of the point of diminishing returns for you. That's a good thing as far as your wallet is concerned, enjoy the 640C.