'Brickwalling' as mentioned around here refers to overload distortion that occurs at a stage in the signal chain before the recorder's meters. Because of that, even if the recorder's input level is set low enough so that the levels appear fine at that point, the signal is already distorted. If you were to look at the waveform in an audio editor, the tops and bottoms would appear as flat plateaus. In essence, any part of the signal that is louder than that level 'hit's the wall'.
The original R-09 (not HR) will distort when the signal being recorded is strong enough to require an input gain setting of about 6 or below on it's scale of 0-30. I don't have the HR version, but it reportedly has similar overload problems with very loud signals. Where that happens on it's scale of 0-50 is the next question to ask. I can't answer that one for you.
My guess is that you are probably either overloading the mics themselves or the input stage of the recorder. Make sure the batteries are fresh and try again, recording something of similar loudness with the bass roll-off filter on the battery box both off and on. If there is no distortion either way, the batteries may have been the culprit. If the bass roll-off eliminates the distortion, then the overload is likely to be happening in the recorder as described above. In that case, note the setting at which distortion occurs and use the low-cut when it gets that loud in the future. If it still distorts regardless of fresh batteries and the bass roll-off, then you are likely exceeding the SPL capacity of the mics themselves.