Are TH215 using Eminence drivers?
Also the TH115 is using B&C 15 if i am correct.
Due to the unusual loading the TH215 had-the Eminance driver is what worked best in that particular alignment.
The TH215 is a discontinued model however. The TH118 is close to the other aspects (size-cost-response etc) but it outperforms the TH215.
The Th115 has always had the B&C 15TBX100 driver. The origional drivers were 8 ohms (and sitll available)-but the standard driver is now 4 ohms.