The drivers are likely 8ohm.....so a 125w @ 4ohm amp would only put about 60w to the horns at full tilt. Still way more than you'll need, but no reason not to use the amp otherwise. It will work fine. Just for reference, I had 50wpc @ 8ohm on my CD2's, the gain turned all the way down and still had to set the level on the processor to -12 (the lowest it would go without completely muting that channel) in order to keep the output in check......just to give you an idea of how little power you need to use with a true HLCD. Really at normal listening levels, you'll use less than 1w of power. At full balls to the wall....horns are capable of enough output to literally deafen you while still being within their rated thermal power handling (which is why I've never understood why HLCD haven't caught on with the SPL crowd, instead of those stupid bullet tweeters). "Big" is subjective. There are a lot of different versions of the CD2's (mylar diaphragm, titanium diaphragm, Neo version ,etc) so differences will be relative. But the CD2 can play lower, probably has a little better performance thru the midrange and may have a little more top end extension depending on the specific driver used. 1200hz, steep slope (24db/oct).