Bluetooth can have a way lower bitrate than say a CD or USB device. I'm not sure what the bitrate is now days or on any specific model, but usually wireless information is lower quality than hard wired information, and has less signal interference problems (because it's hard wired, not traveling through the air like all other signals are too).
So lower bitrate = lower RCA votlage coming out of your radio or processor, whatever you use, less signal voltage going into the amp.
I never set gains with test tones or bluetooth. I always do it by ear with wired in music. If you're playing music, test tones aren't an accurate representation of how the box is going to act throughout it's whole playable bandwidth. Test tones are usually a higher dB level than their musical counterparts, and music isn't a tone, and complex and varying AC waves reproduce very differently than sine AC waves, especially in a ported box, where the electrical properties of the sub can greatly vary, depending on many factors. Too many factors to just trust a test tone burp. I hope I'm replying in the proper context.
