Analog stuff doesn't usually take such leaps forward. A good camera lens or speaker from the 80's is often good now. What's your price point? There's been several really nice releases in the last five years for sure, but there will never be a time (until some perfected automated EQ personalization comes along) that HD650/600 won't be great. Maybe competition could make them overpriced, but even that's unlikely since inflation keeps eating at them.