I know a guy with twin HSU subs and he uses a RANE pro active crossover. The Bass is so deep, tight and controlled.

He now is using two ampos and says the Rane allows him to get great everything!
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Outlaw 1070, Anthem MCA5 II amp.
Sony Ps3
Alon 2 Mk2 Loudspeakers
HSU VTF-2 Mk2 sub x2
VAC PA100/100 Tube Amp
ARC SP16-L tube Preamp
Audio Note Dac 2.1 "B" signature
Furutech E-TP80,
Ascend HTM-200, 340C
Sony KDS-55A2020