I think the decision really depends on how happy you are with the bass from the 1801s. I don't use a subwoofer and am very pleased with the bass. I can, however, certainly understand some desire more bass from their speakers. Also, I am NOT a HT guy, but can certainly understand those who seek this venue of sound reproduction.
I don't think you should sell the drivers and swap them for SCC300's. You will lose $ on this, and the Tempest drivers are very good subwoofers. Sure, the SCC300 probably has a stiffer cone and will likely sound cleaner. However, the Tempest drivers will really pack a wallop. I think you should go with what you have. Build a pair of cabinets for them and put the 15" monsters on both sides of the 1801s. The soundstage will be right and you will be pleased - if you want the bass.
The decision is subjective really. I am sure you can hear the character of the 1801 bass and can hear the character of the Tempest bass. If they match, go for it. Also, if you decide to chase the 15" tempest drivers, please let us the results.
I am sorry for very little solid insight.
Dave