I'm very happy with my btl 18s. I've run them for over a year and a half without issues. They WILL play upper and lower frequencies very well if you want them to, it just depends on the enclosure and setup.
I have memphis 4ks, which imo the BTLs handle great.
An SAZ-3000 will be plenty of power- you COULD put more power on it, but that's plenty to get it moving good.
I had about 7-8 ft net with all the boxes I had just one in, and it worked perfect.
I have them in 5ft each now, and they do sound a bit better on rock, but with a bit of loss in the low end. Like I said, the enclosure will determine how it sounds in the end.
Hope that helps.