Audio sound systems always come down to this...your budget and what your trying to accomplish. I went with Phoenix Gold Elite series components..and at $540.00 they sound fantastic and far exceed any factory speaker Nissan puts in any of thier cars...there are times when the sound just makes the hair on my neck stand up...and I hear things I never heard with my factory system...they play at very high levels with no problems and sound accurate...not muddy or tinny.
http://www.phoenixgold.com/UserImages/ELITE.65cs.jpg