Guys, the great NOS chips are all stale-dated, not current production. Modern chips are higher spec but some people - myself included - like the more analogue sound of the old NOS chips. However the NOS chips are generally more sensitive to synergy with other components to sound their best. My guess is that headphones with strong, even peaky, treble will especially love the HM-602. Headphones that already have softer treble won't suit it so well.
The key point IMHO is that this is not at all an "inexpensive HM-801". It is a completely different beast. The HM-801 is a high-end ultra-modern dual DAC implementation. This is pretty much the polar opposite. And in my view, it is very interesting.