I made small research of AptX and AptX HD audio codecs using music-based df-metric. They were compared with standard/mandatory Bluetooth audio codec SBC. These codecs are used for delivering stereo audio in Bluetooth headphones and speakers.
Conclusions:
(1) Using SNR bits allocation method in SBC encoder gives quite noticeable increase in sound quality (comparable to increase from 201 kbit/s to 229 kbit/s). Bluetooth SIG recommends using of Loudness method as more simple and computationally less demanding. All hardware BT devices that I've measured (USB dongles, smartphones, notebooks) used Loudness method for encoding.
(2) AptX codec used in BT applications is no better than SBC@328. In case of using SNR bits allocation for SBC@328, SBC outperforms AptX. Despite slightly lower algorithmic delay of AptX both SBC and AptX codecs provide the same 100-150ms latency in real-life BT applications. If you hear the difference between SBC and AptX in some BT product, there can be only two explanations - placebo effect or using SBC in Middle or Low Quality modes. It's pretty clear now that AptX was necessary to CSR as a key element of its marketing strategy aimed at effective selling of new CSR BT chips/solutions to manufacturers (with license fees) which in turn effectively sell their "innovative" BT products to consumers. AptX is just a copper-less overpriced audio cable. Audio industry at its best.
(3) AptX HD is high-bitrate version of AptX. It has clearly noticeable increase in sound quality (not dramatic though taking into account the increase in bitrate)
(4) Carefully designed headphones (or speakers) with SBC on board and indicator of quality mode will satisfy 99% of listeners and will be compatible with all BT audio sources on the market.
Details of the research - http://soundexpert.org/news/-/blogs/audio-quality-of-bluetooth-aptx#aptxmeasurements
Illustration by Banksy