If anyone wants to tighten up the bass / dynamics a bit on their HP 150 or 100, here's a method that I call an "external mod", that can be done without any alterations to the headphone itself. Just get 1-2 of these external resistors to stack in front of your headphones, which adds 75 ohms of impedance. Lyova & I have done extensive testing on these, and it seems to tighten up the loose / 'flabby' bass and the dynamics a bit.
Also an external "inline volume limiter" can be used for the same effect. The difference is more subtle rather than 'day & night', and is good for either normal or modded headphone, but I would still recommend the modded version from Tru-Fi speakers (link in my signature). Especially the newest revision for his HP-150 mod - it's a sure shot winner and I liked it even about 30% better than his HP-100 mod
My perception of audio is not quite as great as yours, so I can't say I heard a 30% improvement in dynamics like you did. But, I've always been using impedance (resistor) cables with cheaper headphones that have lower resistance to make up for any channel imbalances (by decreasing the proportion of the difference in impedance to the overall impedance between the two drivers). I'm glad you were there to try that for me though since I, unfortunately, forgot to measure (while I was at the meet) the modified HP150 with and without the resistor cables to see if it made an improvement in the total harmonic distortion ratings (this distortion can be affected by the dynamics of the headphones). So I'm not sure if there is a scientific basis for all this (since I have no personal measurements) other than what I already know about audio (THD, dynamics, electrical impedance, etc.) in general and what Levi told me he heard.
The only reason I prefer resistance cables (/impedance adapters) though over analogue volume limiters, is that analogue volume limiters all tend to have a slight channel imbalance at lower volume levels.
Lyova Margaryan of Tru-Fi Speakers
Edited by Packdemon - 7/23/16 at 3:18pm