it would seem that there are at least two options:
1. a sealed system with high xmax drivers in whatever enclosure and power configuration is required to move the drivers to their limits
replacing the avalanche drivers wouldn't seem to be the best upgrade path. they are good for what you are trying to do. adding to them would likely be the best approach.
2. a large, low-tuned, ported (or passive radiator) setup with a tuning of perhaps 11hz or so in order to pick up the approximately 10db or so of additional output around the tuning frequency and perhaps 3-4db around 20hz.