aptX is superior to AAC because it uses time domain ADPCM instead of perceptual encoding (which are based on psychoacustic models) which are commonly used by MP3, AAC and WMA. This makes aptX more efficient than other lossy codecs.
aptX sounds more like WavPack Hybrid Lossy, since the compression artifacts sounds identical on low frequencies (mild distortion + cassette like hiss)
So if there any AAC file that has been transcoded into aptX, there will be minimal changes to SQ as aptX will not "shave" off very high or low frequencies like what MP3 does, rather it varies the size of the quantization step, to allow further reduction of the required data bandwidth for a given S/N ratio.
So a 16 bit AAC file will be transcoded by the BT device into a 6 or 8 bit ADPCM file - which is 99% indistinguishable from the original file, and that is what you are going to hear when using an aptX device.