Paddy Jackson matched Du Preez's haul with the boot, ensuring a losing bonus point for London Irish.
Ben Spencer ran in two tries and kicked 13 points as Saracens defeated Pool B winners Harlequins 28-21 to reach the semis as the strongest runners-up at the end of the group stage.
Rotimi Segun got over first for Sarries before Spencer took over point-scoring duties, contributing 23.
Joe and Sam Simmonds helped Exeter book a semi-final spot as the best side in Pool C with their tries and kicking in an entertaining 42-19 victory over Bristol at Sandy Park.
Jonny Hill, Jack Maunder, Sam Simmonds and Harry Williams all dotted down before half-time and Dave Dennis did so on the hour mark, with the Simmonds brothers converting every try of the match except Marcus Street's late contribution, for which Gareth Steenson obliged.
Fraser Dingwall's late double saw Northampton resist a Leicester fightback at Welford Road.
Their poor kicking came back to haunt them though as Dingwall dazzled and James Grayson's flawless kicking bolstered a 31-22 Northampton victory. Both outfits missed out on a last-four place, however.
A hat-trick of tries from Marcus Watson boosted Wasps to a crushing 39-6 defeat of Worcester, although the Coventry club also fell short of the last four.
Qualifying had looked like mission impossible for Wasps but Tom Cruse crossed after four minutes before Watson led a second-half onslaught against the Warriors, with Josh Bassett also crossing late on for the hosts.
Worcester's points came from the boot of Duncan Weir, while Billy Searle and Lima Sopoaga kicked reliably for Wasps.