I got a used samson s8 to play my hartke transient attack1400 through using the headphones lineout. It has 2 speaker outputs. 1 has .8mv and 2 has 1.8mv when i read the speaker cable with a voltmeter. Is this a normal reading or is the s8 malfunctioning? Also, is using a pa amp to boost my bass head's volume a bad idea in general?
Those voltages equate to a tiny fraction of a millionth of a watt so no need to worry. It may be that you are actually detecting some AC voltage that your meter is rectifying so some component of the measurement may be a bit of hum or other noise from the amp. There's nothing wrong with using a PA amp as a bass amp. A lot of pro's do away with bass heads altogether and use a bass pre amp and PA slave amps as their preferred amplification. Others just DI into the PA and use a floor monitor or even in-ears for personal monitoring.
Speakers don't like DC voltage as far asI know.