I have the capability of running my subs with a 24db per octave slope with my Alpine 7998 but prefer 12db since it is a more gradual roll off. Since with an 80Hz cutoff I am still reaching out to about 92Hz and downward.
It will sound good with a single SLF-1 and a bypass board, Orion also made a DVX-2 crossover for the pop top series amps as well. It is the equivalent of having a 200 CRX network installed in your amp and also has the ability to run 24db/octave but is kind of confusing to adjust. TristanRules has some I think but the single SLF-1 and bypass will be perfectly fine.
The XTR-2150 is a killer amp, I had one before I got lucky enough to find a GSX-300 which is a hot rodded XTR-2150 in a 24kt gold chassis.