It is possible to overdrive the input of the SubPac's line in port. Some headphone amps are designed for high volumes on stage. This is true of many DJ mixers. When you are using the SubPac without headphones it is possible to have the level clipping the analog input. This can feel great, but sound bad and quickly shorten the life of the SubPac.
The best way to avoid this and other problems related to clipping is to study the gain stages in your signal chain. Adjust the levels so that each stage has a generous amount of dynamic range going into the next. Make sure that none of the stages are choking off or clipping the range of the signal. Remember that a signal with more dynamic range that is turned up will sound louder that one that is clipping, turned down with less dynamic range.
You can test this by focusing on a mix that is compressed and only hitting the upper 3 - 6db below full scale (basically loud the whole time). Set the headphone amp to your stage volume and set the SubPac to your calibration. Make sure you are not clipping the SubPac input. Make mental notes of the sound and feel.
Next, remove the dynamics processing on the master and adjust the mix so it sounds great but only goes up about halfway up the master meter on the average. You will need to turn up the headphone amp and the SubPac to match the feel of the first example. Compare notes.
Most people will prefer the sound and feel of the second example because of the wider dynamic range, less compression, less distortion, fuller sound, etc...
This example will also point out the need for gain staging your DJ sets and having at least two listening levels as outlined by the mastering work of Bob Katz (K system).