We discussed this privately, but I'll answer it here too. The long run XTB-IIR+ was originally built for someone with a farm that had outbuildings located hundreds of feet from his home. The additional signal voltage gave some extra punch on those very long runs. It has also been recommended in a number of other similar installations. The high-output version delivers the same signal voltage at twice the power for installations that have a large number of signal-sucking loads.
The standard XTB-IIR pumps over 25Vpp of signal onto the powerline, and as much as 40Vpp under a lighter load. That represents about 20 times the power of a typical X10 transmitter, and usually delivers plenty of signal over the long runs found in even very large homes unless there is a serious signal sucker near the end of that run.
It is possible that both versions of the XTB-IIR+ may be discontinued before the end of 2016. The PCBs used for that unit are almost gone, and some of the components used for it have become either special order or very expensive. For example, the ceramic capacitor used in the transmitter tank circuit has increased in price from $3 to over $9. The high-current 3.9uH inductor is no longer a stock item at Mouser.
The XTB-IIR PCB was recently revised to accept alternate components (some of which are now being phased out too.) I have also revised the XTB-IIR+ layout to accept alternate components. While a small batch of PCBs from Advanced would have driven the cost up significantly, our supplier in China could make a small batch for under $20 each, including set-up and shipping. So the "+" version may continue to be available at a slightly higher price if there is sufficient interest.