Star ratings on [Mass]Drop and Amazon mean very little to me, given many people who rate and write reviews seldom match the preferences of most people deeply invested in this particular hobby— I’m no gourmand but occasionally write reviews of restaurants. On the off-chance that they do have an interest in “better” audio, that’s no guarantee that they’ll have the same sonic preferences as you either; in my experience, even self-professed neutral-heads, at least those relatively new to the hobby, have divergent beliefs on what constitutes a neutral signature.
Good call on thinking they might not necessarily synergise (I hate this word) well despite both being well-received, high-value performers. I’ve only ever tried the headphones and neither amp nor DAC, but just based on my experience with the K701/HD650 they might benefit from a beefier amplifier (for my tastes K7__ series needs a more relaxed upstream, HD6__ series could use an amp that has good control over the bass).
+1, might have been better to ask before placing an order, unless you’re trying to calibrate baselines with other people, in which case not a bad time to ask.