I get asked the question over and over if the Apollo upgrade is worth the extra cost. In all honesty it is really not up to me to determine that. The TD woofers already have the full copper sleeve on the pole. This is something no other manufacturer does, and it has already drastically taken down the inductance, distortion, and helps pull heat off the coil. The Apollo upgrade then adds 2 giant shorting rings, almost 2lbs of aluminum to the motor structure. In any ordinary driver this would make a massive amount of difference in terms of lowering inductance, distortion, and heatsinking. From anything I have seen, the amount of aluminum used is at least 10x more than the amount of aluminum used for shorting rings in any other driver made anywhere. The coil is essentially surrounded by shorting rings on both the inside and outside. However, because this copper sleeve is so effective to begin with, you will not see a crazy extreme changes in inductance and distortion figures, although these changes are still measurable even at low volumes. As the driver is heated up though is where the biggest difference is seen. Having this massive heat sinking area on the outside of the coil greatly improves the power compression of the woofer. If you only plan to use them for home theater use at moderate volume levels, some people feel it is not a factor and choose not to spend the money on the upgrade. Others think of it as only a moderate increase in price to simply get the best possible option available. I would estimate that out of every 100 drivers, 25-30 of them are sold with the apollo upgrade.