I have a pair of PearlAcoustics Sibelius single drivers speakers. Cabinet design is CRUCIAL: in this case a Voight tube. My personal preference is low clean bass, no window rattling onslaught. The Sibelius itself is perfectly fine (38 Hz, with 32–36 Hz -6 dB), but still added a REL T9 set next to one speaker. It is set at very low volume, don't remember cut-off (crossover) frequency.
I love the coherence a single driver produces. The low bass has very little directionality, so a separate sub works fine.