I've found the difference in sound quality between mid and high cost pre-pro's to be very small to none as far as movies go. The extra cost of the high-end units does begin to show with music only use and most are very good. Just as you would with a dedicated two channel pre-amp, you need to find one that has the sonic signature that you would like in your system. This is not to say that a dedicated two channel pre will not sound better..but you will need a pretty darn good one to outplay some of these pre-pro's as they can do a fine job with music.
Dave
Dave