DH106,the reason that older,straight-wing a/c had a `nose-down ` attitude on approach is that at slower approach speeds ,the flaps are extended,Fowler flaps which move back and down,effectively increasing wing area,This has the effect of moving the aerodynamic `Centre of Pressure`(total LIFT) back,resulting in a `nose down trim..
Modern jets are the same,except that swept wing aircraft also have `leading-edge` flaps/slats/slots,which offset the CoP`s aft movement,and change the approach attitude to more `nose-up`..