Pre-amp with Marantz receiver?

I own a Marantz SR-9300 receiver and am looking to improve the 2-channel performance. I think the amps in the receiver are probably pretty good, and am wondering if there is any value in using a separate pre-amp (which I understand is often the weak point in receivers).

The receiver has inputs called 'Main Amplifier Inputs' which, according to the manual, 'may be used to connect an external source to the the internal amplifiers'. Could I connect a two channel pre-amp to this input? Do you think it would make a noticeable improvement? My source is a Marantz SA8400 sacd/cd player, with redbook fed through a Bel Canto DAC2, to Elac 207.2 speakers.