I'm not sure if 'the more expensive the amp, the better the sound' is true, it's not even true for a lot of headphones. It maybe true for speakers that are very difficult to drive and needed a lot of power, but IEMs in general don't need that. The thing about the UE 18 Pros is that they are made to produce a high sound level without distortion, so they are already quite loud listening straight off the iPod Nano 4G. The E5 is very good for its price, however, since you wanted a sound card for your PC too, maybe you can just get the Fiio E7 so that you can use it as a desktop DAC + headphone amp too, and it's well within your budget.