Setting Gains, Weird Result With Head Unit Setting

Kenwood DMX905S Kicker 46CXA8001 Rather then pay a knowledgeable and experienced professional to perform a "gain tune service", I decided to do the onerous deed myself. I educated myself at U of YouTube. I purchased an inexpensive o-scope, made a speaker-bypass harness with compatible harnesses I bought from Metra, downloaded a 40hz-0db uncompressed test tone to a USB drive and to my phone (for Bluetooth), set each relevant audio setting to my desire, then got to work. The Kenwood permits setting distinct volume gain (in 1db increments) for each input source. The headunit max volume value is "40". When I set the USB, radio and BT input sources to 0db, the headunit clips at "32"--not bad at all. I noticed that lowing input source level by 1db increases clip threshold by two clicks of the volume knob; i.e., -1db = "34"; etc., which suggests I can set this headunit so clipping never occurs at max volume. I felt like an expert. But then... I performed the same procedure the following day. Now the headunit shows clipping at "25" with USB input level set to 0. I changed no other audio settings changed from before. Yet it's obvious something is now switched in such a way that clipping occurs at so comparatively low a volume. I initialized the headunit and started over--exact same result: clips at "25" instead of "32". I'm stumped. Anyone experienced with sort of thing, feel free to offer suggestions!