I can see that. I've tried cross feed in the past and never liked it at all. I don't find it makes something sound like a speaker. It just makes things sound more dull to me. But I could see certain processing improving things. Sort of the way acoustic processing can make stereo speakers really make the impression that sound is coming from behind you. Surround simulation can be surprisingly believable. But unfortunately, for me, my goal has always been to achieve the best quality universally. My phone, my laptop, my ipad. Whatever. So I've always loved the er4. But this filter will let me connect anything to it and get the same eq. So I originally just made the bass boost. And for me it is night and day. Hard to go back. Better than er4xr, better than sr. It just sounds more correct to me.
I'm still playing with the treble. It barely needs anything, but I feel there is room for improvement.
@luisdent Yes, you make a good point. Also, I think our minds adjust to the presentation of headphones. Without spatial DSP, headphones sound very 'hard-panned' to myself, with little 'centre' information. But, I also listen off my phone a lot. I use nPlayer, which is very resource efficient on iPhone (i have an old 6). It has DTS_X, but on the headphone setting and 'traditional' it is very light, and doesn't seem to interfere too much with signal integrity. It is a happy mid-point. Definitely recommend nPlayer if you are on iOS, it has been well-supported for years. I use it for FLAC, and lossy formats. Interfaces takes a bit to get used to (double tapping on album image etc) but it is a very solid player.
Also, you've been a huge proponent of the ER4SR so thank you! It was a factor in getting them. The ER4SR is one of my favourites. The isolation alone is essential, and creates a listening experience that is a bit alien. In some ways you are hearing music unfiltered, uninterrupted from your individual ear shape, HRTF, etc. It really is a reference in this regard.