Digital Designs subs are made by TC sounds who make very good subs. Unfortunately, they DD subs are designed for SPL (large ported enclosures tuned at a high freq). It would be difficult to get real good SQ out of them, IMO. I haven't heard their components sets. They could be very good.
About the component sets. MB Quarts are pretty nice but you will still want to cut the bass at around 80-100 hz or so. and 100 watts would be pushing the limits of the capabilities. I have 50 watts going to mine and I wouldn't use too much more without the MusicComp (sp??) crossovers.
I would guess that you are right about the 6dB slope on the tweeter causing a problem. I would go with at least 12db. 18 and 24 db slopes are very good in theory but are extensive to make (if made with good quality components. The crossovers are easily the most expensive pieces in a component set.
What kind are you currently using?
You could always make you own crossover...

[This message has been edited by dnewma04 (edited January 30, 2001).]