I was running mine off an A600.2 (150x2 @ 4 ohms), and I had my gains practically turned all the way down. With my Alpine 9811 on 20, they were pretty much maxed out. If I left them cranked like that for long, the tweeters would cut out and then come right back as soon as I turned down the volume. Eventually, I blew one of the tweeters anyway. At moderate levels, they sounded okay. And I guess that just says a lot about their efficiency. You can get them VERY loud without a whole lot of power. One other thing - I had the head unit crossover on my Alpine set at 120. At 80, the speakers would make a loud popping sound at decent volume. They would probably be fine at 90 or 100, but my head unit jumped straight from 80 to 120. My Bostons play 80 and up, with lots of power, at high volumes, with no complaints whatsoever. When I get my 701 hooked up, I may even go a tad bit lower with them. Just be careful with the XRs, and enjoy!