amp 4 channel bridging

I got this used legacy 600 watt 4 channel amp for a decent price so I can't complain that it's not the best. Anyways I got one pionner 12" sub which I bridged the two channels together to get 300 watts. Someone side that if I don't have anything hooked up to the other channels 1/2  that 3/4 will put out max power of 600 watts. I Think that I am only getting 300 watts. Can anyone conform if this is true?

If 4 channels give you 600 watts, then 2 channels give you 300 watts....as long as the impedance of the load (the sub) is rated the same as the amplifier rating.  If you bridge one sub (4 ohms) to the front two channels of a 4 channel amp,rated at 75 X 4 @ 4 ohms,  you will get 300 watts.  If you bridge the sub to the rear channels, you will also get 300 watts.  Use two subs to take advantage of using a 4 channel amp. Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

First, max wattage means nothing, talk about RMS if you must talk about wattage.  Second, whoever told you that was wrong.  Your amp is probably more like 150x2 bridged, if that.  Even if you leave channels 1 and 2 open, 3/4 will still only put out that 150x1 rms bridged.  New Project: 2003 Pathfinder