You could add an external DAC and use the BAT as the transport until you decide to upgrade the transport. One good thing about using a CDP as a transport is if the DAC needs to be serviced you still have a CDP, but this is just a bonus, not a reason. If you like the sound of tube output there several tubed DACs available and many of these are discussed in another current thread on Audiogon.
It won't necessarily be a waste if you like the sound of an external DAC better than the one box BAT, but if the improvement is slight you have to weight the cost/benefit yourself. I used an Audio Note DAC with an Electrocompaniet CDP for a while and the improvement was dramatic. I now use a dedicated transport in place of the Electo.
If you want a recommendation I have two: the Audio Note balanced DACs (I have the 2.1x, the baby of the bunch) and a Wavelength Audio Cosine DAC. Both of these are zero sampling tube output DACs and I love the sound of them. Very smooth, dynamic, 3 dimensional, highly musical. I can't speak as clearly about the Cosine DAC as I have not compared it to other CDPs yet, but the AN DAC is unlike any other digital I've heard. It is the last stop on the digital train for me.
See if you can borrow a DAC to pair with your BAT and compare the sound to using the BAT alone.
