Yes, the DAC is a digital to analog converter so the data will only go through a DAC once, otherwise it would have to go through an ADC to convert it back to the digital domain. In this case the DAC used will be in the BT headphones or headphone amp with a BT receiver (ie after the BT signal is received). What will happen is that your audio will be transcoded from it's current format (eg MP3 / FLAC) straight to the AptX format for more efficient use of the BT bandwidth. This will probably lead to some loss of quality as AptX lossless is not available for BT. The BT standard does support transmission using the original encoding, but this doesn't seem to be something that manufacturers advertise if they support or not unfortunately.