When configuring voice processors such as the dbx 286A/s (or any other device with a similar configuration) – there is always an optimal preamp level setting or sweet spot for the connected microphone. Basically – your mic needs to be properly driven at the preamp stage in order to pass sufficient gain with low inherent noise and ample headroom throughout the device and thru it’s downstream processing modules.
In general, intra-device Drive based Compressors are designed to elevate the module input gain as the setting is increased. In doing so the dynamic range of the passing signal will be decreased. This often results in an elevation of the noise floor that was nonexistent prior to the compression stage.
Please note: After initial preamp optimization, this setting should remain static. The preamp level control should NOT be used for gain staging or compression noise floor compensation! In essence improper preamp gain will hinder the effectiveness of downstream intra-device processing.
My recommendation for optimal signal to noise: set the preamp gain accordingly. Apply intra-device processing. Lastly, use the OUTPUT gain for any necessary gain staging or compensation. This will have no effect on the initial (and hopefully optimized) mic input setting as well as the subsequent processed signal passing through the device.