Anyone who has already taken the plunge on active studio monitors would do well to at least take a look at the sort of audio interface designed to partner them. $150 is a little bit on the low side but there is still loads of choice. My shortlist at that price would include.
Native Instruments Audio 2 http://www.native-instruments.com/en/products/traktor/dj-audio-interfaces/traktor-audio-2-mk2/specifications/
Two stereo output channels each with it's own volume control. These channels can be the same or different. Works with phones and tablets too.
Focusrite 2i2 http://uk.focusrite.com/usb-audio-interfaces/scarlett-2i2/specifications
Again separate volume controls for phones and active speakers. Adds balanced TRS outs for your monitors. Stereo pair in in case you ever want to connect an good microphone, instrument or any line level source. Worth looking at the 2i4 and 6i6 is your budget can stand it.
EMU 0404 http://www.creative.com/emu/products/product.aspx?pid=15185
Golden oldie now but still excellent and adds optical and co-ax S/PDIF I/O.