Re:Zoom H4n as usb interface?
As you see I also have the same interface. I find it is very transparent and have never noticed and colour added to the sound. I use input 9 or 10 at about 12 o clock. Can't remember if the pad is in or out. But if you don't like the sound of your bass it would be the bass it self that has issues, all interfaces would seemingly record it the same once the gain is set properly.
You best option would be to check out better pick ups, I have Seymour Duncan's on one and Dimarzio on the other. They are passive, active Pick up's for me always sound kinda distorted.
Other options are run the bass though a good quality pre amp.