CON: If something is radically different, it can be a good thing sometimes IMO. But that is not the case with 3.Ai. The mid-frequency notch is more prominent that that of 4.Ai, killing the overall spectral balance, and making the user effectively hearing-impaired at 3 kHz to 4 kHz range; A huge crossover network design flaw on Heir Audio's end. Moreover, there are more issues that need to be addressed: inverted polarity, high distortion, narrow bandwidth, and slow sub-bass.
Just like 4.ai, changing the insertion depth does not alter the sound of 4.Ai that much, even with 3~4mm distance. Of course, there is not much of high frequency to be altered to begin with.
Adding an extra resistance to 3.Ai is definitely not recommended- it will even further skew the frequency response, worse than the case with 4.Ai.
Heir Audio sells an upgrade cable,Magnus 1, for $149. Since the manufacturer says it has more silver content, it should lower the overall impedance of the IEM, and increase the output SPL due to improved efficiency in electroacoustic transduction. Since 3.Ai is more efficient than 4.Ai, the resulting relative output difference is actually larger.
And just as expected, my random $25 furutech cable from Ebay performs better than Magnus 1.