Yea, the Hsu STF-1, or the SVS PB10-ISD should work. You could go even cheaper possibly. But for quality, I wouldn't drop below those two. The SVS will be a bit more expensive.
He'll still have quite a gap in frequency I think between what.. 80hz and around 200hz? I forget where the roll-off on the 901's is.
In order to smooth that he he'll have to ditch those 2 901's