The U5 is a "studio DI" and is NOT intended to handle phantom power. I have one and don't use it in situations where there may be question. I do use it live, but only when I know the situation will be ok (pro PA, pro sound person, and I ALWAYS talk to the sound persons BEFORE I plug it in...). I've heard of others who did use one with phantom power turned on and ended up having to send it back to Avalon for repair. Now you *can* lift the ground, but with no DC blocking caps on the output, you could hit the output transistors pretty hard with 48VDC when you plug in a hot channel. There are cases where it could be harmful, but not always.
The mic/line outputs are "electronically balanced", but it is 100% discrete Class A and does not use opamps or caps in the signal path. Putting 48VDC on the collectors will certainly cause problems. The collectors are held to 0V with a servo (probably opamp-based), but it's unlikely that they can compensate for 48V @ 10-15mA.
I'm in the process of trading my U5 for a GT Brick (one more session with the U5, then the trade deal is done), but I've often thought I would get a 1:1 Jensen line transformer to run in cases where I didn't want to risk phantom power on the output. I have a Radial JDI for that, though...
BTW - the Brick is already transformer-balanced, so there's no need to worry about phantom power on the output! Use it at church and be done with it.
Oh, and many DIs that are intended for studio use do not handle phantom power - at least not well. Both my SWR Bass 350 and my Epifani UL502 DIs sound great, but don't do phantom power (opamp balanced). The only way to block DC is with capacitors in the signal path, which can be detrimental to tone. Not always, but the fewer the better (they can cause phase distortions and other ill-effects). Any "electronically balanced" output (opamp or discrete) will either require DC-blocking caps and other components to bleed the DC off - essentially a filter - or they won't handle phantom power well. Transformer balanced outputs are a different story. I've heard some people rant and rave over how "stupid" it is to NOT use DC-blocking caps. Most of those people only do live work, where it's an issue. Far more studio engineers are worried about tiny details of the signal and chain and therefore, care about if or not there are DC-blocking caps or opamps in a signal path. The U5 is designed for
those people, so it caters to their needs.
