I have the same exact model. But what you should do is buy two subs that are rated at around 150-200 watts then run a parallel with it. To run parallel, hook the positive and negative of each sub to each other, then run it to your amp. The zx300.1 can push 300 watts at 2 ohms.
But to answer your other question, you should go with a sub that is ment to run on a lower wattage, than the higher. This will make it sound better, and will be louder.
What i did is i got 3 subs. Two are Rockford Folsgate subs 8 ohms 150 watts each RMS ( model - punch p1S810 ) . Then i have an MTX that runs 250 watts at 4 ohms RMS ( model - JackHammer JH45 ) .So i ran all the subs in a parallel, ran all the positive and negatives of the subs together, and hooked it up to the amp. HITS HARD