May I recommend the Nobosound Amplifier that has a left and right sound bar so you can differentiate the stereo output of Left vs Right? It costs a reasonable $40 and they offer coupons to lower the price on USA's Amazon.

Oh - so you don't plug them in 3.5mm audio to activate they activate via ambient noise and powered by USB? You mentioned this before but I was not envisioning that at all. So thank you for the video/pictures.

Here is an affordable headphone/speaker amp with a built in Left/Right vu meter (I think it is in the useful links page as well).