... I'm using a pair of 2-way bookshelfs rated at 4 Ohms ... However, I'm not completely satisfied with the high-frequency response of the speakers and would like to add another pair that excel with midrange and treble ... I'm only using the "A" set ouf outputs, so I assume that the next pair should be on "B" ...

... Do I need to get another pair also rated at 4 Ohm ? ... Is it okay to pair a 4 Ohm and 8 Ohm on "A+B", respectively ? ... Also, does anyone have a reccomendation for a nice bookshelf with excellent upper-midrange and treble response that will not be too-demanding on the 2150 considering I am using 2 pairs of speakers ? ...

... And lastly, are there any negative sonic implications in using 2 pairs on A+B instead of just one ? ... Were the "A+B" terminals intended for bi-wiring only ? ...