I think it is pretty common to use the TacT with an external preamp (I do, so it must be common, right?). It is also common to use it with an external DAC, and then you need to have the preamp to follow the DAC.
If you can swing it, I would try the TacT for awhile before you give the BAT the boot, both to see if you like it in the system and then to see if you like it as a preamp as well as a room correction system.
If you can swing it, I would try the TacT for awhile before you give the BAT the boot, both to see if you like it in the system and then to see if you like it as a preamp as well as a room correction system.