Depends on the preamp. XLR output is pointless, as DD has only RCA inputs (except for one XLR LFE input for HT.) A Y-connector bridging L/R outputs of preamp works only if the preamp outputs are internally buffered. Otherwise outputs to main amp will also be bridged into mono.
Because my Atma-Sphere preamp briefly emits DC off-set when it's turned on, I buffer line-level outputs to DD sub with Jensen Isomax coupling transformers optimized for LF use. But this is not a typical configuration.
Because my Atma-Sphere preamp briefly emits DC off-set when it's turned on, I buffer line-level outputs to DD sub with Jensen Isomax coupling transformers optimized for LF use. But this is not a typical configuration.