Is -12db an absolute number?

The levels in FCP are measured in the Decibel scale called Decibel FullScale (dbFS). The top of this scale is 0 dbFS. This is the absolute maximum level which can be handled in the digital domain, there are no positive levels in dbFS scale. All levels are negative relative to the top of the scale. While in the analog world overdriving (more than 0dbu) continually degrades the signal up to the saturation point, in the digital world any levels which would exceed 0 dbFS lead to unacceptable distortion (clipping). Contrary to old school practice, it's not at all desirable to peak as close as possible to 0 dbFS.

If your peaks at -12 to -6 are distorted then it would seem to be the incoming signal that carried the distortion and was not a result of recording those levels into FCP itself

you'll find excellent info on this, and far more in depth knowledge than I could hope to offer, in Bob Katz's articles on the Digital Domain website.
here's a link to one, go, read and then take a look at the others ... then come back and be our digital audio expert!

http://www.digido.com/bob-katz/level-practices-part-1.html

cheers
Andy

What were you using to record the event?
Were you taking +4 dBu line level directly into your video recording device via balanced XLR, or using a matching box to provide a consumer -10 db level for your device's unbalanced inputs (RCA inputs)?

It's important to know the audio specs of this device to come up with the correct answer.

One possibility, assuming that the signal was leaving his board clean without peak distortion, could be that the signal was over-loading your device's input stage before it gets to the gain control on your recording device.
If this was the case then the signal would have to be padded down before it enters your recording device.
One clue to this being the case would be that you had to set your input level much lower than normal.

Without knowing what reference level he was sending to you, and the input capabilities or you recording device, no really accurate answer can be given to this question.

Which model of Beachtek adapter?
Did you have a mic level feed from the house audio guy?

Levels on your camera can peak to -1 db full scale without distortion.
If your audio source is carefully monitored, recording at a level where your peaks only ever hit -12 db will lower your S/N ratio.

This is really tricky to control during a live event as the house sound guy knows he can constantly hit +3 VU on his analogue mixer without noticable distortion.
You on the other hand are in the digital world where there is no "headroom".

Unless you work very closely with the sound guy even setting your level to -12 full scale won't protect you.
A good mixer can output short peaks to +22 dBm without distorting.
In this situation a limiter between the mixer and your inputs would help.
Ideally, you also want to be present during rehearsals so you can anticipate problem passages.

BTW, during my 30 year career as audio recording engineer with the Canadian Broadcasting Corporation, i have made many hundreds of live recordings, simulcasts, CDs, and lived in both analog and digital worlds. So I fully appreciate your situation.

The DXA-4 has had excellent reviews.
Being a passive device there's no preamp that could overload.

Reading up on your camcorder i saw that it has a peak limiter built in that's always on.
Plus an AGC which can be turned on or off.

Over at the bealecorner this report on audio with this camcorder:

http://www.bealecorner.com/fx1/index.html

"Note: in manual audio levels mode (both mic and line level) the FX1 has an always-active internal limiter which starts around -8 dB full-scale and limits the average recorded level to about -5 dB full-scale. I view the presence of this limiter (which cannot be switched off, as far as I know) as a good thing from a production standpoint, as it extends the useful dynamic range of the audio inputs by about 20 dB before severe clipping starts. However, it can complicate audio distortion testing."

Based on this and a lack of any complaints about audio distortion in all the reports I read, I'd say that the distortion most likely originated from the house sound mixer.