If you want headphones to work properly on a phone or iPod or something they need to have a headphone amp and DAC.
Basically - there's no point in spending a lot on headphones if you dont drive them properly, and there's no point in spending a lot on a soundcard, DAC or amp if you don't have good enough headphones for them to power.
Having said that - my headphones work *okay* through my iPad and phone. I say okay because they sound alright, but due to a lack of amp they can't get particularly loud.
You have a few options because of this:
Get something like a D2X. This soundcard doesnt have a headphone amp - so you could get:
http://www.amazon.co.uk/FiiO-E5-BK-...1?s=electronics&ie=UTF8&qid=1347880439&sr=1-1
to amp up the headphones.
That could then be used on your phone or iPod too to give you a better audio experience on that.
Or...
You could just accept that your audio experience is not going to be as good on your phone and get the Essence STX - then save up for a DAC/Headphone amp especially for the phone or iPod like:
http://www.amazon.co.uk/FiiO-Portable-Headphone-DAC-Amplifier/dp/B003N0XDT4
Either way - these will both probably cost more than your £100 budget - and even more with the headphones - which as I said you need to spend a fair bit on to make everything I've just said worthwhile.