Are you saying, that with the same cables, wiring setup and adjustments the old one never did this, but the used you bought does it ?

Like all reputable equipment manufacturers, Presonus will be making continual small changes to a model as it goes through its production life. These changes are triggered by things such as particular components becoming obsolete or no longer available in the package that was originally designed in, lower-cost replacements being available for certain parts, and, significantly, legislation changing so that equipment has to meet ever-tightening restrictions on interference generation and susceptibility.

It is likely that your (failed) unit and the second-hand replacement you bought come from different stages of the product's manufacturing life, and that, by chance, your original unit was less susceptible to mobile phone interference than the replacement. I don't think it indicates a particular fault in the replacement unit, but what it does flag up is that you and all the people you work with should adhere to the general studio rule that requires mobile phones to be turned fully off when you are working within 10m of audio gear.

I've never had any issues with CP interference. Just lucky, I guess.

How much importance should be placed on balanced lines, or is cell interference a different kind of beast?

I'm only assuming that it's RFI?

Yes, it is RFI, but it's different from effects due to TV or radio breakthrough. What something like a local 3W 1GHz transmitter does is spread RF energy into screens of cables and through leaky earth bonding of connectors, and then, once inside a chassis, gets itself partially rectified on any convenient semiconductor junction. What you hear as the characteristic phone pulsing noise is a representation of the envelope of the transmitter waveform, effectively in d.c. form. Balancing as understood at audio frequencies plays no part in this type of problem.

Keep your phone away from any audio gear!

I used to have an iPhone 5 and it would cause a little bit of buzz once in a while if it was sitting right on top of my KRK V4 monitors. But now I've downgraded to a Nokia flip phone and I can't have it anywhere near my monitors without them going crazy. :/

Thanks guys. I was really just concerned that something may be wrong with the Presonus. Guess I just have to keep my phone out of the room.