Pre-Amp for Class D Amp

My brother use a pair Bel Canto 1000M(500W@8 ohm) class D power amp to drive a pair B&W 802D speakers. He is happy with the 1000M. The pre-amp is a Bel Canto Pre3VB. He want to improve sound. Is there any suggestion a better pre-amp to match 1000M? Thanks.