Hyperthreading increases efficiency and processor throughput for a PC running a large number of services and processes. Under such conditions, with newer processors, it is reasonable to recommend leaving hyperthreading turned on.
However, for a highly optimized audio-PC running a much smaller number of services and processes, there may be an advantage, with respect to sound quality, to having hyperthreading turned off. This may be why Audiophil recommends turning hyperthreading off when using the Audiophile Optimizer (AO manual, page 9).
Thus, as with other factors in computer audio, whether it is beneficial for sound quality to have hyperthreading turned on or off will likely depend on the individual system in use. As I mentioned previously, and Windows X has just emphasized, it's up to individal users to try both ways and determine which works best for them with their systems.
Control and Audio PCs
Dell Inspiron 15R SE laptops with Intel Core i7-3612QM (2.1 GHz) and 16 (Control-PC) or 8 (Audio-PC) Gb DDR3 1600 MHz RAM
OS: Windows Server 2016 optimized (via Fidelizer 7.4 and Pro 8.1, Audiophile Optimizer 2.20 beta 6, Computer Audio Design scripts) to Virtual Core Mode on Control-PC and Audio-PC (http://jplay.eu/foru...ws-server-2016/ page 9, post #162); Additional Optimization: Process Lasso Server Edition 18.104.22.1682; File Manager: Q-Dir 6.99.2; Library Manager: JRiver MC19; Renderer: JPLAY 6.2 with ULTRAstream engine in JPLAYStreamer mode; Server: Minimserver 0.8.5.2 with Avconv transcoding; Control Point: Kinsky 4.4.5 / Upplay 1.2.8
Control-PC > SUPRA Cat 8 Ethernet Cable > Audio-PC > Chord Silver Plus USB Cable > Uptone Audio Regen (Amber) > Curious Cable USB Regen Link > Luxman DA-06 > Chord Chameleon Plus XLR Interconnects > Luxman L-505u > Van Den Hul Teatrack Speaker Cables > Bryston Model T Signature Speakers with external crossovers
Seagate 3-Tb ext HDD (USB to Control PC) plus two 3-Tb Seagate backups