The acoustics of your rooms are very important, so if you have a horrible sounding room then your in trouble!
studio monitors are designed to have a flat response over the whole FQ range. although this is not always the case, and there is not really a 'flat' (all monitor do have a slight harmonic characteristic to them) if that makes sense....
Reference is a key word. you need to know how your monitors sound inside out, (play your fave album over and over) your effectively referencing against the existing available material. as well as ruling out unwanted FQ within the individual sounds, ie, taking anything under 200k away from a guitar as that may be a nasty bassy hum, which is not want you want competing with your bass FQ. (very layman's terms)
Headphones are for tracking your stereo field initially and monitoring high pressure sounds, ei a Kick drum, although if your studio is lovely and nice sounding i think think your need them much in the mixing process
The stereo field on headphones is very lets say 'exaggerated', and most listener will be experiencing the music on HI FI speakers, which will be complimenting the sound, and sweetening!
dont use hi fi speakers for monitoring!
Headphones.... IF YOU WANT TO MIX ETC ON HEADPHONES... try this... [url="http://www.focusrite.com/products/audio_interfaces/vrm_box/"]http://www.focusrite.com/products/audio_interfaces/vrm_box/[/url]
mastering too... FYI is for MASTERING ENGINEERS! its a black art and the finest producer don't master there own music! you can sent your compleate mixes to RAK EMI [url="http://rakmastering.com/"]http://rakmastering.com/[/url]
hope this helps.