I can't find this explained anywhere, but I'm leaning towards -10db simply because other monitors in this range go to about 41 or 42 Hz.
EDIT: Well it appears I'm correct, and PreSonus measures at -10db instead of +/-3db:
"The Eris E8 features an 8-inch, Kevlar low-frequency transducer, driven by a 75 watt, Class AB power amplifier; and a 1-inch (25 mm), silk-dome, high-frequency tweeter with a 65W, Class AB amplifier. It can deliver up to 105 dB SPL, peak. Frequency response is rated at 45 Hz to 22 kHz."
That's from an old announcement ad posted by PreSonus themselves, and it appears they are being dishonest about the frequency response.