This is absolutely the best sounding direct injection box I have ever used on bass guitar. I typically pair it with a faster, cleaner preamp.
For instance I can use the REDDI to fatten the sound of an otherwise pedestrian sound. I pair it with the Brent Averill 312 preamp. I simple plug directly into the REDDI and out to the 312.
The gain staging is dependent upon whether you use active or passive pickups in your bass.
If using passive pickups on a bass such as a Fender Precision, you may need to apply more gain from the REDDI.
If using a a bass with active electronics, such as the Music Man Stingray, you won't need as much gain.
Structure the gain in a way that incorporates your bass controls, the REDDI and your preamp.
With the Stingray I tend to have the REDDI's gain at the one o'clock position and the 312 at the eleven/one o'clock position.
With the P-Bass I'll have the REDDI's gain at the one/three o'clock position and the 312 at the eleven/one o'clock position.
I like to keep the preamp in a somewhat neutral position while supplying the additive tone from the REDDI.
This provides a suitable gain staging throughout the chain.
Experiment with different basses, preamps and the various tone controls on your bass.
The REDDI adds a smooth, yet growly tone to the bass. It also cuts through a dense rock mix with less processing.