The preamp part of the 727 itself is ill-designed; therefore, with a better preamp, you can actually get a much fuller and more punchy sound on the 009. Not saying the 727's preamp is hardly listenable, it's just that you can easily hear a difference with a preamp hooked up to a 727 in direct mode.
There is no preamp in the 727. Only thing the switch does is defeat the volume control which you can also do by turning it to its max position. Only two plausible reasons you should hear any difference, impedance mismatch which the source can't deal with and you hear the coloration of the preamps circuitry.