*** NEWS FLASH ***
I discovered something about the BL-03 today, that was previously unknown by the community.
I was carefully examining the BL-03, and I noticed that there is a fully functional bass vent right at the 2-pin plug.
This bass vent can get COVERED UP and sealed when the stock cable is used (or a banned brand that makes an upgrade cable using the stock plug ends):
However, when a generic 2-pin cable is used, the bass vent is fully UNCOVERED. This is due to the way the generic cable sits on top of the stock socket.
I tested this with different cables, and sure enough there is a difference. This difference is not due to the cable itself, but rather the vent being covered vs uncovered. I think this can explain at least why some people were hearing different BL-03 tuning, the (untrue) rumors of the different colors sounding different, etc. Some people said it had too much low end, others said it was too little, etc.
You can even test this with the stock cable, by partially unplugging the stock cable so that it unseals the vent. This proves that a stock vs 'upgrade' cable is not reason, but rather the bass vent.
The BL-05 has no such bass vent, and there is significant difference in the quantity of bass between the BL-03 vs BL-05:
Sure, the BL-05 uses a different shell and newer generation of driver, which does have some influence on the difference between the BL-05 vs BL-03. But the lack of bass vent on the BL-05 is a significant contributor on the BL-05 having less low end vs the BL-03.