Sennheiser HD 650 vs Sennheiser PXC 550

The sound signature is near neutral but with a little emphasis on the lower frequencies. The bass is slightly forward with good punch and tightness. Explosions will have good rumble, without overpowering the other frequencies.
The mids are warm and clear, adding extra body to the sound. Game dialogue will be easy to hear.
Highs have good clarity, but are not forward in any way, so harshness is minimized.

The H650 is popular for its near neutral sound, which makes it quite versatile. The headphones can give a great casual listening, gaming, and movie watching experience yet are accurate enough for pro audio mixing. They are also lightweight and comfortable so you won't have any issues wearing them for hours.

Clamping force is not too tight. The ear pads and headband feature memory foam which are covered by velour material. The pads feel really soft on the head and allow airflow for minimized heat build-up/sweat.
The headband also features similar cushioning materials and minimizes pressure spots on the head.

The open ear cup design allows the sound to blend with your surroundings, giving it a natural width. It will also be easier to identify the direction of sounds, which can really help immersion.

The HD-650s do not feel heavy at all. They are usable for long music listening, movie or gaming sessions without feeling pain or strain on their head and neck.

While the cable connectors are proprietary, they can easily be removed from the ear cups with a firm pull. The popularity of these headphones means that there are a lot of aftermarket cables that can be bought.

The feature works really well in preventing outside noise from intruding into your music listening experience. Also, there are very little to no reports of a feeling of pressure in the ears when the feature is active.

The sound tuning of these headphones is focused on the lower frequencies. The bass is quite powerful with good sub-bass extension. Users who listen to a lot of hip-hop, dance and bass-heavy music will certainly enjoy the oomph that these headphones can give. The mids sound natural and smooth, yet the lower regions of these frequencies can be overwhelmed by the powerful bass. The highs are crisp and clear though, which prevents the sound from getting too muddy.

Soft foam and leather are used in the interior and exterior of the ear pads respectively which makes them feel really soft and smooth on the head. The same material choices can be found in the headband.

The Smart Pause feature detects if you are removing or putting on your headphones. These gestures will trigger the music to pause or play automatically.

When in wireless mode, you do not need to use buttons in order to control the functions of the headphones. The right ear cup lets you control the headphones through touch gestures. You can swipe left or right for next or previous track, swipe up or down to increase or decrease the volume or tap on the ear cup to pause or play track as well as answer or end calls.

You can set just how much noise is cancelled by the headphones, from maximum active noise cancelling, to an ambient mode which allows some sound into the headphones. You can also turn the feature off entirely.

These headphones are quite effective at preventing sound leakage, and also passively block out noise really well. This is achieved by the closed ear cup design along with the effective seal from the leather-surfaced ear pads.

You can twist the ear cups and then fold them into the headband for a smaller, more travel friendly size. In this state, it should take up a lot less space inside your bag for less hassle when used on the go.

Sennheiser offers the CapTune smartphone app which allows users to see the battery level of the headphones, adjust the headphones' EQ, and control other functions.

The case is made from a tough fabric material which should protect the headphones from minor bumps and scratches.