A good rule of thumb is to spend ~20% of your budget on your Interface/DAC. So you are looking around $200 to add to your 305s, headphones and the tube amp you already have.
That's actually a more than adequate budget. So you can get something decent with transparent conversion, flexible facilities and reliable software. Wide choice too.
I'll use the Focusrite 2i4 as an example but other brands such as Native Instruments, MOTU and Roland are also excellent value.
http://uk.focusrite.com/usb-audio-interfaces/scarlett-2i4/specifications
Connect your digital source via USB, connect the active monitors via the balanced outputs 1-2 and control the volume from the front panel, connect the headphone amp using the unbal outputs 3-4, these are fixed level outputs but your device has it's own volume control. You might not think you want the input ADC functionality right now but you will soon find a use for them (BT receiver dongle is very handy, lets you connect your phone or tablet without wires).
The cool feature about this set up is that the two stereo output pairs (1-2 & 4-4) can be different! Headphone amp with separate control could also be useful either as a spare or for when you want clean, accurate output. Even if you don't use those features at first the unit isn't going to cost you more than hi-fi equivalents. Have fun.