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Was the Crucial Ballistix RAM one of the compatible upgrades listed here: https://www.crucial.com/compatible-upgrade-for/dell/xps-8940

Yes, I chose the Crucial model from that compatibility list. Apparently, they didn't test it out themselves. So don't be misled by that list.

I contacted their technical support by email. Their support is not technical at all. They simply forwad some links in their website, which is not related with my issue at all.  Seems the technical people respond to email once per day after breakfast.

I submitted a survey and rated their technical 0 start. Then later, a product manager contacted me, asked me to "turn on something called XMPin the BIOS", "I downloaded the user manual, but unfortunately I don't see XMP as an option, you need to contact the  computer manufacturer ...."
Come on, I clearly explained Dell's motherboard have no XMP option in my very first email. Not only XPS 8940, I never heard of any Dell motherboard has XMP option.

So far, never heard back again from Crucial.

@Fei Crane I am beginning to think that this is a problem with the XPS 8940. Since the XPS does not use the XMP, it depends upon the SPD information. It may be that the memory you have tried does not have the information required to set the speed at 2933 MHz. If you look up the specs of the Corsair Vengeance memory you used you would see that the SPD is 2133 MHz which is exactly the speed you got. The HyperX memory specs do not list the SPD and usually, with Crucial memory, the speed rating is the SPD although it too is not listed in the specs. With most memory manufacturers if they indicate a speed it is the tested speed using the XMP.

Ram speed is determined by FSB of the cpu and XMP profiles etc.

You cant mix speed and vendor and profile and expect it to work correctly

Adding faster ram does not change the Profile for a given processor.

Along with speed ratings, RAM also has ratings for timing.

This appears as a string of numbers such as 9-9-9-24.

SPD stands for Serial Presence Detect, meaning that the motherboard expects the RAM to correspond to one of the JEDEC profiles or XMP Profiles.  You cannot mix speed and profiles and vendors with good results.

Ram speed is mostly marketing.  You are falling victim to the MHZ Myth.

https://www.youtube.com/watch?v=D_Yt4vSZKVk

Of course it is Dell's mother board issue. From the information I got so far, Dell's mother board cannot provide enough power to 1.35V Rams. 1.2V Ram would work fine. I am in the middle of testing out.

What I don't like Crucial is: their website claims the ram is compatible with XPS 8940, but it is not. And their technical support is not professional at all.

Overall, feel XPS is a shabby computer. I hesitated to DIY a computer by myself, or by a existing one.  Haven't DIY computers for years, now found out everything is RGB, and turning off RGB is a hassle, so finally just bought one to make it simple. I know Dell won't use the best parts, but still disappointed by what I got.

I have watched that specific Linus video and other videos before. I still want to my ram run at 2933 instead of 2400. I don't call that a "failing victim"

I am not aware that you can set the RAM voltage in the BIOS on the XPS 8940 motherboard although I am pretty sure the issue is not that there is not enough power to supply another 0.15V. Perhaps you should have brought memory that is rated with an SPD of 3200 MHz at an SPD voltage of 1.2V  While the SPD specs of the Crucial Ballistix memory are not listed the voltage listed is 1.35V. Perhaps you should have purchased Crucial 64GB kit DDR4-3200 (CT2K32G4DFD832A) which has a rated voltage of 1.2V. For the Corsair memory you tried, the SPD speed is 2133 MHz and the SPD voltage is 1.2V. 

I am not why you are critical of Crucial and not Corsair. Crucial's memory worked but not at the speed you wanted just like Corsair's memory. Crucial's issue may be the rated voltage and Corsair's issue is that the rated speed is different than the SPD speed.