[Minstrel59]

Which in ear monitors (less than £300) give:

Massive isolation
Transparency
Good resolution
A soft treble
Minimum fatigue
Warm sounding (not bright and sharp)

Thank you

 

[billybob_jcv]

I would probably be looking at Westone or Shure. They take a wide variety of ear tips (actually they are interchangeable between them) and might generally match your sound description to some degree.

 

[davidcotton]

Shure 215
Klipsch x10 (or I) might be of some use.
Iheadphones.co.uk have some Westone 4rs down to £300 that might be right up your street as well
 
For £300 that would get you a couple of budget customs from the likes of Custom Art in eu or Cosmic Ears here in the uk.  You would have to factor in for ear impressions (around £30-50 depending on where you go)but that would leave you a budget of around £200 or so.  Depends how much less (much I guess) down on £300 you want to go.