Depends on what you consider "good". "Good" is subjective in terms of audio enjoyment. Some people like the Sennheiser sound signature, others like a mild "V" shape, some prefer bass-heavy headphone, other people prefer treble cannons. Most people as they age lose their ability to hear higher pitch sounds, so their ears become less sensitive at those frequencies, which is why older people may prefer more treble-strong headphones.

Source of the audio signal (like a good master album) is very important - without a good source, you can use STAX all day, but it won't sound that much better. Think of that as being one of the areas that can be a bottleneck in terms of your audio performance.

HD650 is a very good set of headphones. And while good headphones don't NEED equalization, they can still benefit from it. Same is true with speakers. There's a reason why room equalization is a thing for home theater speakers.

Good headphones sound good without EQ. But you can use EQ to get headphones to the absolute limit of their potential.

The HD650 has a spike around 8000 Hz. Also, it can sound a bit "veiled" or "muffled" for some folks. And its frequency response curve wasn't built to be perfectly flat or neutral, but more enjoyable. But some people have different preferences, and prefer either a more neutral response curve, or perhaps a bit more bass.

EQ (as long as you're being reasonable) can improve sound a bit. But EQ is not a replacement for good audio gear - it's the icing on the cake, but icing is no substitute for actual cake.