I personally use a Subpac to mix my low end with Audio Technica ATH-M50 headphones. I used to have a subwoofer as part of my set up but that just caused a lot of issues from poor room acoustics. Your 8" monitors may be too large for your room as well since they have a frequency response down to 35 Hz. (I use 5" monitors since I'm in a small room).
For the kick/bass relationship, tuning your kick to the key of the track will help clear up a lot of things. I also tend to keep my kick short, so I don't have two big sub elements fighting for space.