Naggots,
If you have Druid 4-08 with Mundorf silver oil in the network, you're quite close to Soul Superfly, and of course stereo Mini-Methods will outperform Superfly on bass - albeit at expense of space and complication.
The FRD in Soul has wider horizontal dispersion and more vivid, punchier dynamic behavior than what you get from Druid 4-08. It's a refinement of the Zu FRD, so essential character isn't changed. But it's clearly an evolution that makes the FRD better, and not in any way worse. The tone density is similar between the two, but the newer FRD is burstier and sounds faster on transient attack. The motor is stronger, the cone a little lighter than older versions, and the phase plug is not only cylindrical but the detail on the face of the cylinder is essential to its role. The last trace of darkness in the Druid's sound -- much removed by the 4-08 upgrade -- is now gone. In many rooms, given the trouble most rooms pose in the deep bass region, Soul's 30Hz lower flat limit will be both easier to manage and more truthful than the deeper subs, but if you have that sorted, OK. But sans subs, bass from the Soul is in all ways better than what's possible from the Druid form factor.
I plan on replacing my Druids with Superfly Soul, though I don't use subwoofers. The price is right and the improvement is of sufficient increment to be worthwhile to me, for the system I use Druids in. But it is an evolutionary upgrade from your speakers, not a dramatic one.
Phil