Are the R1600T's the ones OCUK sold around 10 years ago for £50? If so I had some of those back in the day, they were pretty good for that money.
Have you put your R1600T's on desk stands and got them to ear level? I would be doing this first before upgrading speakers.
£150 is a limited budget, but would look at these.
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If it was myself I would buy a pair of Adam Audio T5V's for around £240 (yes you'll have to pay more), but you'll get a proper upgrade and your sound card is enough to do them justice. You will need a pair of quality shielded RCA's cables also. You can get 3 meters pairs made up with Van Damme microphone cable for around £30. Again all this is over budget but you'll thank me if you can stretch to it.