Your absolutely correct on the outputs of the different types. The 1/4" line outs on the pod are much hotter ( obviously +4 dbu ) than the other such outputs. In the amp ( little switch that says line / amp ) setting my guess is that it is a -10 db setting to accomodate the standard -10 db in most guitar amps and the XLR outs on the pod are sent at MIC level. This is meant to be sent directly to a mixer where it will be gained up to unity. As a goal for setting up things this way, I use this approach.

1. Most mixers show you were the theoryetical unity gain is achieved. I set the HA ( head amp ) gain knob to that spot.

2. I then turn the channel volume ( The actual knob with volume written under it ) down after the effect is chosen. I mean on the POD,  not on the mixer. This level should already be down. 

3. I bring it back up while playing with the PFL switch on the mixer engaged and stop when the level reaches unity.

For those less knowledgable about pro audio I will explain a bit. Unity is the expression used to descibe that there is neither attenuation, nor amplification of the incoming signal.  The HA ( head amp ) is a fixed gain potential device. That means that the knob is mearly an attenuation knob. The strength of the incoming signal will determine the position that that knob will be when UNITY is acheived. In other words a low signal will require less attenuation and the HA gain will be turned pretty far up in order to achieve unity gain. This is why I set the HA gain knob to it's unity gain position ( if you mixer shows you that ). I do this because this setting is the optimal position for the most amount of gain and least amount of noise. The Unity position marked on most mixers is mearly the spot were unity gain would be achieved if the incoming signal was at unity it'self. That means that the reference signal coming in at unity ( or essentially 0 db ) would be unity at that HA gain setting as well. To achieve unity gain simply PFL the repective channel ( to see it's signal strength prior to the channels fader ) and adjust the HA gain until the meter blinks to 0 db. This is the optimal way to achieve low noise and good channel fader resolution. You will also notice that the level meter on the side has numbers and at zero or 0 db it says normal. This is the unity gain level and is the level at which you should get all inputs to be at when they are PFL'd. There is another type of metering called AFL, or After fader listen. If you use AFL your meter level will be what the channel is at including it's fader position. I.E. if the fader is all the way down you will not see any level at the meter. Unity gain on the channels fader is that larger white line in the lightly shaded area. It should also show a zero or a U. Refer to the picture and you will see the U located at the 12:00 position on this particular mixers gain knob and the U located on the channels fader. The norm or 0 db is located on the last green led of the level meter.