I recently purchased a pair of Shure SRH840 headphones. I use these for both listening on the train and for watching movies at home so I don't disturb the kids/wife at night.
I have found the volume on these lacking when using my Samsung Galaxy Captivate android phone, though the quality is decent especially since it supports lossless music. I also find the volume very lacking on my home a/v receiver a Yamaha RX-V1900 using the included 1/4" to 3.5mm adapter, after switching to headphones I have to turn the volume WAY up to get a reasonable level. Volume and sound is great directly out of a PC headphone jack though.
From my reading I presume this means it would benefit from a dedicated headphone amp. Can you please recommend if I should consider a portable/stationary DAC and/or AMP for the following 2 scenarios, and if I should consider separate hardware for each (let's assume $100-200 budget):
1. Android phone for portable use on the train.
2. Yamaha RX-V1900 watching movies and occasional music, fed from a Home Theater PC that's connected via HDMI to the a/v receiver.