would it be possible to plug PA speakers into the headphone out on an amp? would it be safe?
While you can do it, I wouldn't. First of all, the headphone output from an amp is not going to be a very large signal - in other words, it's not strong enough to drive a loudspeaker. Also, you run the risk of an impedance mismatch. Most headphones are rated higher than 8 ohms and your PA speakers are likely to be rated for 4, 8 or 16 ohms. Not sure how tolerant it would be of this mismatch, but it would suck to find out that it wasn't and you smoked the headphone amp.
Does your amp have an optional speaker out? some amps will let you bypass the stock speaker to connect it to a cabinet. I'm not sure if this helps at all.
no, i have to use a 75watt line 6, i cant use my primary amp (2 speakers, 120 watts) because the feedback is horrendous and the place im gonnna play at wont mic my amp
If your amp has a speaker emulated line output (not sure if the spider does or not tbh) you could plug it directly into the PA via a line input, but not the PA speakers, because speakers take high voltage signals that a headphone output does not supply.