Greetings, as you know, Etymotic ER-4P model may be converted to "S" by adding a 75 Ohm resistor in series. It turns out that HF2/3/5 are not much different (you may take a look at Headroom FR and impedance graphs to see). Here are the values I've tried: 0 Ohm (stock condition) - loud, but too much lows and mids. Not boomy nor muddy, just lacks air. 16 Ohm - much better tonal balance, but still too warm for bass-heavy tracks. Enough volume to never push my iPhone beyond 50%. I had this setup for a few months. 68 Ohm - almost perfect, yet a little bit harsh at higher highs. 56 Ohm - not much different from 68 Ohms, just a little warmer and louder. This is my current final setting. Now, please take a look at stock frequency response: A series resistor combined with coil makes a simple LR filter thus pushing higher frequencies up. The trouble is, upper treble is pushed up the most, but there already is a spike. I found a solution: a resonant RLC filter. After doing some SPICE simulation I figured the going values: 10 Ohm in parallel with 4,7uF capacitor - almost no volume loss, well controled lows and Gradoish vocal presence. May try lower resistor value to tame the lower treble boost. Regretfully I didn't manage to fit the whole circuit inside the mic housing, so I'm staying with 56 Ohm resistors for a while. I highly recommend the RC mod to all ER4P and HF-2/3/5 users.