I realize you are talking about reducing the internal volume by partitioning, but I did want to comment on the subject of larger cabinet volume, nonetheless. One can gain bass extension with a larger cabinet, provided internal standing waves in the midrange are mitigated. But what I suggest instead is to use flanking subs, which serve three purposes:
1. They provide bass extension
2. They provide baffle step mitigation
3. They provide mitigation of SBIR and higher-frequency room modes.