Hy, as Andrew(Murph)pointed out, spending more money on amplifiers would only be of benefit to you if at brief peaks your present Yamaha wasn't providing enough power for your M60s(unlikely). If more power was in fact needed at those peak moments, you'd be able to play them about 3dB louder with the doubled maximum power capacity that you're considering. At all other times there'd be no audible difference. Keep in mind that unused amplifier headroom is simply that: unused.

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Enjoy the music, not the equipment.