In addition to level matching, if you are interested purely in sound quality. it is better to set both units to two channel 'pure direct' or something like that and play some high quality music CDs, instead of using logic 7, DVD movies, or other processed sound effects.
Anyway, if it is too late, then why not just enjoy your 745 and assume HK sounds better regardless of the reasons. One fact to keep in mind though, based on more than one lab measurements, the lower model Denon 3805 (37.5 lbs) has significantly higher power output into both 8 ohms and 4 ohms (higher current) than the equivalent HK 630 (also 41 lbs). I would think it is unlikely that the 645 has more power output than the 4306. My be something else was causing your unit to sound harsh at high volume.