External power is rated at 5V-500mA, which also happens to be the maximum power a USB 2.x port should be able to deliver. Unfortunately it isn’t always as simple as that. Easiest way would be to do a simple test. Just add the adapter and listen if it’s louder.
Also it depends on what outputs you use. Especially if you are taking RCA outputs and running them (through adapters or adapter cables) to XLR inputs. RCA is -officially- a lot lower in output than XLR/Jack (balanced).
Apart from that, some sound cards/controllers are quieter than others. I had a Numark iDJ Pro which was WAY loud for example. Louder than any other controller I ever owned. I don’t have hands-on experience wit the VCI 300 though, so hopefully one our readers does and can chime in here.
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