Should I use the volume control on the front of the HD3 Next Gen's or can I adjust from system volume on my computer?

If the left speaker is in an easy to reach location, you can certainly use the physical knob to set volume. If not though, you can set the HD3 Next Gen at a fixed volume and adjust from your source device. Around 50% is a good starting point, but you'll need to experiment to find what setting works best to give you a good overall volume, and a smooth ramp up.