The VS has specially designed hi-z inputs for instruments that can work just fine. But if needed, a DI is almost a must for guitar/bass with passive pickups, plugged directly in. Active pickups (those instruments that require a battery) can do okay without a DI, I think.
Line level instruments (keyboards...) don't require a DI, since the VS has line-to-mic level attenuators.
I've been advised before that a passive DI should be used with active pickups and an active DI works best with passive pickups. It's been quite awhile since I put that to the test, but maybe a good thing to consider.
I'm sure others here have strong opinions based on more regular use with their VS machines.
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uptildawn
