I'm amazed that so many put up with the volume control as supplied. (1) The easy/lazy way to have at least a reasonable volume range is to attenuate the pot's output by using appropriate resistors before the pot see
www.goldpt.com/mods.html. (2) Remove the pot (really easy) and sub with a standard Alps Blue 50K shunted with Z foils see
www.world-designs/forum, look for the FAQ forum and 'shunting the pot. (3) just remove the existing pot and use an external volume control like - Stereo Coffee.
Whatever type of h/phones you use up rating the o/put caps from 30uF per side will give a real solid bass to the overall sound. Removing the PCB to do this is easy enough but it's hard to replace because the Tx wires are so short. Much simpler is to destroy the existing Chinese caps but making damn sure you leave the very thin legs intact. You can then solder new caps to these with no degradation in sound as long as your nifty with a soldering iron.
The only effective way to remove the hum is the Fitz/Pink Floyd way. These 3 mods elevate the DV to an entirely new level.