The night before last EA5DOM (1525km) was copied 45 times on 472kHz with the preamp in circuit, so the FT817 was at maximum sensitivity. My best report of him was -16dB S/N.
Last night he was copied again but without the preamp. Last night I copied him 35 times with a best sensitivity of -19dB S/N.
Although conditions may have been different, I think the 472kHz preamp is worth having. It allows signals too weak to copy to be successfully decoded. Incidentally, EA5DOM was copied as late as 0740z today at -26dB S/N without the preamp on 472kHz WSPR.