Dopps:
That's why I asked what you're recording.
The line out from your bass drum to DI? Your bass drum has a line out? I'm probably not the only person confused by that statement, so let me ask:
(1) Are you miking your bass drum? If so, it's ok to run the mic directly into the VF. You would probably want to compress/limit it but that's a different issue.
(2) Do you have synth drums? If you have synth drums, it's ok to run the signal directly into the VF unless your kit has some sort of speaker setup. Again, avoid tapping speaking/power outputs unless you have the proper interfaces.
As for the guitars,
(3) Are you miking the cabinets or taking a signal from the amps? Again, DON'T TAP ANY SPEAKER OUTPUTS. EXTENSION SPEAKERS, SPEAKER LEADS INTO YOUR VF unless you have the proper interfaces. Many DI boxes will not perform a speaker/power tap correctly, and even those that can need to be set/used properly.
(4) Line outs, headphone outs, direct outs, insert outs, etc. (i.e., line-level signal outputs) may not provide the signal that you think you are sending or want to hear. If not implemented properly by the manufacterer or misapplied by the user, they can make the signal sound brittle or harsh but they should not harm the VF inputs. These types of signals usually benefit from DI and RedBox application. Check the owners manual of the amp. If it states that the recommended recording output is "normalized" or "compensated", than the manufacturer has, to some degree addressed, the issue of providing a signal that attempts to mimic the miked sound of their amp.
(5) Recorded bass almost always benefits from DI. Tapping an input AND an amp line out signal/DI output is a good way to go. The same precautions as stated above apply for tapping an amp signal. If you're going to close mic a bass cabinet, there's always the old SM57 standby,
a Behringer XM8500 (with or without the ball---a really inexpensive 57/58 alternative), or: Audix D6, Sennheiser e602, AKG D112.
(6) Beyond miking, vintage amps require more dedicated devices to record direct. They usually didn't have the recording/mixing options included or considered in the design of the electronics.
I hope this helped.
Good luck,
Paj
8^)