As expected, the argument "implementation..." also popped up in such discussion. Logically though, let's assume the following:
Specs:
DAC A better than DAC B, say by 6 dB in both THD+N and DNR
Implementation B better than A, also by 6 dB in both THD and DNR
You can see that in theory, DAC A + implementation scheme A = DAC B + implementation B, but just as clearly, we can see that:
DAC A + Implementation scheme B would be superior, audible or not is a different matter/topic.
Now, let's take consider a real example, RX-A3080 has the ES9026 Pro, THD+N: -110 dB, DNR: 124 dB
compared to the A-S801's ES9010k2M's THD+N: -106, DNR: 116
One would think that the AVR RX-A3080's DAC plus implementation should, better or worse, come very close to that of the A-S801, and that's assuming the A-S801's implementation is in fact done in some sort of superior way, such as balanced/differentially wired. Based on what I saw in the schematics, I doubt it was wired this way.
I really think this so called implementation being more important than using a better DAC chip is BS most of the time in the low to mid range gear. Then again, even if the gear is truly high end, then if an excellent implementation scheme is used, such as being fully balanced/differential all the way (e.g. the ADTG hated Denon AVP..
),why wouldn't they go with the best available DAC chip
too?? So I would say probably most of the time (not always,/always exceptions..) to claim that "don't worry about the chip, its implementation that counts blablabla... are excuses, and/or hearsay that got repeated enough times to be taken as facts. Just my 0.00002 cents, feel free to ignore.
To the OP's question, the ES9038Q2M has better specs in both THD+N and DNR, but the AK4490's numbers are already excellent. The AKM chip was in fact a flagship just a few years back. In terms of implementation, it would be hard to compare without the necessary information such as a full set of schematics, parts list, and the ability to analyze such information. It may be better to just base your decision on other factors, such as features, look, prices and bench measurements.