Given your budget, you are spending too much on the external sound card and not enough on the speakers if your aim is to maximise sound quality. Your requirements would seem well suited to 2.0 or 2.1 near field monitors. Whether you are better off with or without a sub depends on the kind of music you favour. Monitors with active crossovers and one amplifier per drive unit are usually of higher sound quality compared with equivalently priced passive + amplifier and is a neater package to have around a desk. If you can raise the monitors a foot or two above the desk, place at 1-2 metres and avoid things like large computer screens in the way to diffract the sound you will get a clear sound with precise images a bit like headphones.

Welcome Suds.

I see that you started with cans. That can really give you a lot of bang for the bucks. Could you describe your headphone rig so that we understand what you're used to?

Dave