Firstly, to sound as good as Index speakers, you might need something that lets you have some natural HRTF and crosstalk. Using the very same Valve Index drivers in a closed earcups with pads probably won’t give an experience as good as once floating over ears. Closing the Valve drivers will make you feel low frequencies (bass) are louder but in detriement of frequency response qualities and overall 3D experience. Ear cups are acting like small rooms where the sound bounces around ; they will add some resonance, distorsion and coloration that is why Vavle decided to bypass them.
That is what Vavle says about its audio solution : “Mid-high frequency sound fidelity is very important. Binaural simulations rely on subtle changes in tonal coloration (1kH-8kHz) to convey the position of a sound source relative to the player. If a listening device adds its own muffled frequency coloration, this will interfere with the player’s ability to localize sound.” https://www.valvesoftware.com/en/index/deep-dive/ear-speakers
So what you are looking for is not much a “great/fun sound” than some kind of “accurate sound”. You are looking for a device able to reproduce/resolve frequencies with as much fidelity as your budget allows. Sometimes the better headset is the one that is precise enough to let you ear all the artifacts, compressions and imperfections of your mp3 source…
Maybe you can try some in-ear (great ChiFi) solutions? There are different budgets :
- BLON BL03 (22$)
- Tin T2 Plus (45$)
- ThieAudio Legacy 3 (130$)
The last two will be the more accurate ones. These are said to have a sound signature close to Sennheiser HD600.
In an other budget you can get the Raal SR1a earfield ribbon monitors : https://youtu.be/LLJ1XjXeGfc