amp
the main amp wil sound more full and smooth, more like the wattage it is rated at, the yamaha on it's finest day could never produce 100 watts X anything 20hz to 20kzh.... the real two cahnnel amp you have will... Believe me on this, especially for music the secondary 100x2 amp will sing, the yamaha 100x5 or whatever will choke the speaks to death at high volume levels, possibly even clip and toast the coils in your drivers. Unless you are talking about a huge 4k + receiver, then an external power amp is your best bet for the main speakers.