The QuietComfort 20 headphones have been around for years and years; and they’re essentailly an in-ear alterative to Bose’s QuietComfort 25. They offer the same great active noise-cancellation that the company is known for, just in a traditional wired and in-ear form factor. The QuietComfort 20 can also be switched to an “Aware” (aka ambient) mode, so you can better hear the world around you.

Open back usually results in more natural sound, but they leak sound in and out. This means you probably can’t use them in a library or a plane flight since the sound of your music will leak out and disturb the people around you. You also can’t use them on loud public places since noise from the outside will disturb your music. Therefore, even though open back would give you more natural reproduction, for these reasons their use is limited to mostly at home or in the office (if you have your own space that is).
Released in 2017, the Bose QuietComfort 35 (Series I) are essentially the exact same headphones as the Series II. They have the same design, feel, sound quality and noise-canceling skills. The difference is that the Series I don’t have Google Assistant built-in and a dedicated button on the left ear cup to activate it. If you don’t care about talking to a virtual assistant while wearing your headphones, which allows you to play/pause music or skip tracks via a verbal command, then Series I or Series II shouldn’t matter to you. The catch is that the Series I is more difficult to find online and they aren’t usually that much cheaper than the Series II.

Just a few hours of burn-in today – I don’t expect much change with Tesla-quality drivers etc. The treble is recessed almost as much as the Philips M1 I had, kind-of a worst-case scenario. So I took out my most minimal non-peaky non-bright non-sibilant headphone – the B&O H6, and even though it doesn’t sound the same as the T51p because of the H6’s “light” midrange, I wanted to get a sense of how much the T51p was recessed below a very minimal treble. My Foobar2000 settings were +2 at 2.5, +4 at 3.5, +2 at 5, +4 at 7, +6 at 10, 14, and 20 khz. Normally I wouldn’t do the dip at 5 khz, but the T51p has a nasty 10 db peak around 5 khz, which makes it difficult for portable use without a customizable equalizer. Without a treble boost it sounds very boomy as well as muffled. I can understand Beyer going to a darker sound with more bass – in fact I thought it was a move in the right direction. But they need to cut that (resonant?) peak around 5 khz. I compared to several other headphones and none of those were anything like that.
Gaming headsets are headphones that have a microphone attached and allow users to speak to other people and hear them with the same device. In fact, gaming headsets have other practical applications as well. You can use them in business or personal situations when you want to speak to people on a computer using a video phone program or social media chat room. Although these work for music applications, they're more suited for conversational clarity.