looks like a decent amp, a good all-around performer however not their top end. I would think it should not have an issue powering 2 tower speakers, so I would guess that if you are not happy with the overall sound, then you are down to one of 2 issues:
-something along the way is not set correctly, such as a hp/lp filter setting on either the deck or the amp not allowing a full range signal to the speakers affecting their sound range or
-speakers are not of the quality you are looking for..

if you have the time and inclination, you might try hooking up the towers to 2 of the channels of the 700.5 and see if the overall sound changes, but I would guess that it's the speakers.
bud of mine has a pair of skylon cans on his tige that he bought off ebay. they were priced somewhat cheap and he only bought them for the cans and plans on replacing the speakers this year. they are 2 years old, have maybe 40 hrs on them, stored indoors and sound like absolute crap. no low end or midrange at all, almost all muddy vocals and high end.

good luck. I would think either amp should push a set of towers to a decent level