Valve put a lot of R&D into the off-ear headphone solution shipped with the Index. Unfortunately one of the big compromises they made was that it has absolutely zero response below 40hz. If you record the bottom string of a drop-D bass guitar, you simply wont hear it on a stock Index. Unsurprisingly, explosions lack the usual expected rumble, and music games like Beat Saber sound weak with no punch on the EDM tracks. This was a show-stopper for me – I immediately permanently removed the off-ear headphones and used some quality IEM’s instead. Sorry Valve for the hard feedback: compromising on bass was a bad call.

Heres my first attempt at resolving the audio problem with this otherwise remarkable VR headset. I went with the Koss PortaPro mod route, to get a Rift CV1-like sound.

• The quest strap bracket from here (thanks meisenhut31) was printed at 1.3x size to fit the index.
• I ended up not using the included holes on the 3d print but drilling my own, as a bit more angle was required. This extra angle is shown in a below photo. I might revise the design and redo this properly later.
• The easiest way to dismantle the Porta Pros is angle grind off the metal head strap. Otherwise you’ll need to drill out a pop rivet under the label and re-build somehow.
• The tension mechanism in the Porta Pros works nicely – once the headset is donned,  pushing the headphones inwards causes the mechanism to click twice as it ratchets in.
• The wiring was uneventful apart from needing to find a very thin 3.5mm headphone jack – the right-angled one that comes with the porta pros wont work.
• I had to file back the bracket on the left side to accommodate the headset cable retention mount.
• As-yet untested with glasses. I’ll update this later.

The amount of bass below 100hz generated by these headphones is tightly related to how firm they are pressing on your ears. Getting the correct angle is critical here.  The following screenshot is my initial tune in Equalizer APO to get the Porta Pros sounding flat. Yes I realise this will clip badly if pushed hard. The Index DAC and headphone amp does seem to have a bit of headroom, but you will need to play it by ear and turn the volume down a bit if you can hear clipping.

The reason for gaining it up this high was to get an acceptable level at 100% volume in Beat Saber.  It sounds great, with plenty of volume linear down to 30hz and drops off to barely audible at 20hz. On my set of Porta Pros i have a slight air flutter at 25-40hz from the right side only. It doesn’t bother me but goes to show they’re very far from perfect. It should also be noted the amount of boost that was required on the top end to get these sounding linear. My set sounded very muffled by default.

It’s tempting to have another crack at the angle of the mounting to get a firmer fitting and reduce the correction at 22hz, but the extra ear pressure would be uncomfortable. As it is, this feels exactly like a Rift CV1 – very comfortable.

I’m about 90% happy with how this turned out. I’d like to find a better sounding on-ear headphone (linear 20hz-20khz without pressing hard on ears) with a spring mechanism and efficient enough to not run into the headphone amp ceiling. All in all, after EQ, the Porta Pros do sound considerably better than the Valve Index off-ear’s, but there’s room for improvement.