Magnepans do WELL recorded vocals amazingly well to the point that you will think that the person singing is right in front of you. Acoustic music is another strength.

I would not worry about your Onkyo amp. The Maggies will sound just fine. They require a lot of clean power, especially if you like to listen at high volumes. With better quality amps you will get better resolution, deeper and punchier bass. You wont know any different unless you were doing an AB comparison between amps.

Just make sure you use a heavy guage speaker wire, 14g or better. I used 16g xp monster on my MMG's and they heated up at higher volumes. I use 10g with my MG 1.6QR's.