Are the Mbox 3 Pro line inputs too hot?

[bcent15]

Here's my issue: I have an API lunchbox with a 512c preamp installed connected into a line input of my Mbox 3 pro via a single XLR female to 1/4" TRS cable (Lunchbox to Mbox respectively). The line input is set to +4 on the Mbox. When my input signal on the preamp reads 0VU, the input level on the Mbox and in Pro Tools 10 reads about -6dBfs. That's too hot, it should read about -18dBfs, correct? Depending on the system, 0VU should be at least somewhere between -20dBfs and -15dBfs. This is frustrating because this setup won't allow me to open up my preamp as much as I 'd like to. According to the Mbox technology guide, the line inputs of the Mbox 3 do indeed bypass the internal preamps: http://www.avid.com/static/resources...logy_Guide.pdf

Does anyone know what is up with this level mismatch?