iPhone decodes Vorbis codec and decompresses it to 16-bit 44.1 KHz PCM (Pulse Code Modulation) stream. The PCM stream (side note: if you look at AIFF or WAV that is essentially is PCM in a "container" and that in its uncompressed form is 1411 kbps) is then compressed to SBC standards which is only 328 kbps (for a 44.1KHz sample rate), only 23.2 % of whole information. That 23.2% is what goes to your 1000X which will then be converted to sound through the 1000X's DAC (digital to analog converter)
See here:
http://www.couthit.com/codec-sbc.asp
For AAC playback, iPhone DOES NOT decode the AAC. Instead, it transmits the AAC file without prior decoding whatsoever via Bluetooth (therefore no losses) which the processor inside your 1000X will decode and decompress to 1411 kbps 16-bit PCM and the DAC inside will convert 100% of the information in that AAC file instead of 23.2% of it to sound, giving you equal sound to playing an AAC file wired!
That's the reason some Bluetooth headphones don't support AAC.
AptX works on a similar fashion as AAC where the file that is transmitted is not decoded to PCM prior to sending over Bluetooth.
A quote from Wikipedia states, "The aptX audio codec is available for consumer and automotive wireless audio applications,
notably the real-time streaming of lossy stereo audio over the Bluetooth A2DP connection/pairing between a "source" device (such as a smartphone, tablet or laptop) and a "sink" accessory (namely a Bluetooth stereo speaker, headset or headphones). aptX technology must be incorporated in both transmitter and receiver to derive the sonic benefits of aptX audio coding over the default sub-band coding (SBC) mandated by the Bluetooth standard."
AptX is not limited to AAC, therefore if you play an MP3 which is lossy, that file will be transmitted via Bluetooth without decoding so no information is lost during the transit, provided that the receiver can decode the file and convert it to sound. I believe OGG vorbis is also supported through AptX especially when those Bluetooth speakers feature Spotify connect or something along those lines.
Hope that helps explaining the quality difference between Bluetooth codecs