EBMM Preamp (in a Luke III)… voltage sensitivity?

Just wondering, how sensitive is the preamp to voltage dropping from an aging 9V battery? Does the guitar stay the same until it dies, or does the preamp start getting distorted, or what?

On a whim, because I leave guitars plugged in alot, I just checked the 9V in my Luke III. It was down to 7.4V, I put in a fresh battery (reading 9.9V), and I don’t think I hear an y difference. Just wondering what tom listen for to know the battery is tired…