Soul Supreme compacts Druid 5. Soul 6 compacts the sound of Druid 6. with the newer concentric driver. Druid 5 was a relaxed, easygoing, amp-tolerant speaker, more open and elastic than Druid 4. Druid 6 is strikingly more vivid, energetic, revealing and requires more thought about the gear train that comes before it, particularly the amplification. Soul Supreme and Soul 6 have a corresponding relationship.
One would expect a $6000 (Soul 6) speaker to both be more vivid and revealing than a $3800 (Soul Supreme now) one, and so it is. That difference is an opportunity but it also means that while both can work well in your room, Soul Supreme is less likely to prompt one or more changes in associated gear when plopped into an existing system.