With windows 8.1, Yes you must enable "Balanced (Recommended)" in "Power options" or if high performance is used  go in to "change advanced power settings" and click the + next to "Processor power management" and change "minimum processor state" to 0 or 5% either will work click apply then OK. As far as your bios settings are concerned most mobo bios default optimized settings are sufficient and should not need any corrections to get the low power state but YMMV 

To really save on power usage you need a MB that supports off set voltage control. Your CPU is still at peak voltage even when the CPU frequency is throttled down.

I'm not familiar with the 5960x or the x99 platform in general but I'd say it has something to do with load. If you are at 100% CPU load on all cores and it's still not at the max default value then something is throttled in bios or the Windows 8.1>Power Options> Advanced> Processor Mgmt > Maximum Proc State,  may not be set to 100% but I digress the operating freq for that proc is 3.46 and Max turbo frequency is the maximum single core frequency at which the processor is capable of operating using Intel Turbo Boost Technology. It may be that you will not see 3.73 on all cores when all are maxed out, rather only when a single core work load hits a single core it has the potential to reach this max turbo freq as demand dictates. Now there maybe a way through bios to boost all core to the max turbo but someone else will have to chime in to solve that.