How much amp you need is also governed by the XO. If you're using passive XOs, and not bi-amping, then you can absolutely benefit from a big amp with plenty of reserves to adequately drive the low end, and the resistance of attenuation for SPL-balanced compression drivers. Personally, I've recently tried bi-amping my 816 bass horns and small-format compression drivers, all Altec high-sensitivity drivers, using an active XO, Onkyo 75wpc SS amp for the LF and 17wpc tube amp for the HF. Surprisingly, combining everything onto a passive XO and driving the whole thing with the Onkyo resulted in a better overall sound. I don't have to turn the amp up hardly at all (with 7:00 is min, 5:00 is max, I rarely need more than 8:00)...