I have a set of the Dunlavy SC-IVs along with about every other Dunlavy model that was ever made. Yes, going to larger Dunlavys will give you more bass foundation, but (loving clean deep bass as I do) I have still felt compelled to add subwoofers. John Dunlavy did design and manufacture a few Dunlavy subs in various sizes. I came across a passive Dunlavy sub that was in an SC-IV enclosure with four 10" drivers. Using a separate crossover and 200 Watt (high current) Mono amp with SC-Vs has brought bass performance to a level I wasn't sure was even possible.
My advice is keep the Dunlavys and keep an eye open for Dunlavy subs to match. It will probably take about 4K to get it right with arrangement similar what I've mentioned above.