My M-Audio Fast Track Pro defaults to 16 bit when I use my Line6 POD HD500X's SPDIF output. My DAW (Sonar Platinum) shows Audio Driver bit depth at 24 in Preferences, but it's greyed out.
 
Question 1: What's up with the greyed out Audio Driver bit depth at 24 in Preferences?
 
Question 2: After reading tech stuff about bit depth on the internet until my brain started to bleed, the consensus seems to be that yes, 16 vs 24 makes a difference, but it's only detectable by uber audiophiles with super human hearing. The main effect is on storage space required for recordings, and CD's are 16 bit. Opinions?

1. This is the only option provided by the driver
 

You seem to be confusing the long standing and opinionated arguments about sampling rates higher than the Nyquist frequency with the issue about bit depth. The bit depth encodes the dynamic range of the sampled audio, the sampling rate encodes the frequency. High sampling rates can accurately encode frequencies that are too high for humans to hear, or rather too high for anyone but bats and audiophiles to hear. But bit depth encodes the power of the signal i. e. the number of decibels that can be encoded without clipping. There is a very practical use of a higher bit depth, and that is to avoid clipping, which even us normal people can easily hear. Given the choice most recordists will choose higher than 16 bits because it gives them some wiggle room in the strength of the recorded and processed signal. As a rule of thumb a bit of depth provides about 6 dB of encoding, and so 16 bits will provide more than enough dynamic range for music at ~96 dB. If your 0 dBspl is the threshold of hearing, the threshold of pain is about 120 dBspl, and except for club music most music is not supposed to hurt. But it is not uncommon to capture a loud sound and amplify it through a couple of stages to the point that your recording is in the red with 16 bits. Careful attention to recording levels and gain stages can produce a good recording at 16 bits, but the extra bits at the top end make clipping less likely. And if the source you are actually recording has a range from 0 to 96 dB, you are likely to be picking up electrical noise at the quiet points adding hiss or crackles where you intend to have silence. So the advantage to more bits is in what you will not hear, rather than what you will. Twenty to twenty four bits is probably plenty to avoid most problems. But I guarantee that most everyone can appreciate the difference between a sound at 96 dB and one at 140. Then again at 140 dBspl distortion is probably the least objectionable issue with the sound. 

scook
1. This is the only option provided by the driver
 

If the driver's Control Panel defaults to 16, shouldn't it say 16?
 
On the bit depth issue, I'll have to re-read what I read. Since clipping hasn't been an issue, I guess 16 will be OK until I do. Thanks for the detailed explanation.

rd2rk
If the driver's Control Panel defaults to 16, shouldn't it say 16?

This would be a question for the developers of the driver and control panel software. AFAIK, SONAR provides the options supplied by the driver. My guess is the driver supplies a 24bit data stream regardless of the control panel setting.

According to the manual your Pod should be outputting a signal at 24 bit depth via the SPDIF.
Again according to the manual, the Fast Track should be able to get 24 bit depth, but you will sacrifice a couple of the channels. Have you tried disabling the channels that you do not need and setting the bit depth to 24 in the Fast Track? You will also have fewer options for sample rate depending on bit depth and number of channels you activate, but that should not be a problem unless you are making music for bats.
 
"Configurations
At 16-bit operation, the Fast Track Pro operates as a four-in, four-out device at a maximum sample rate of 48kHz, with analog and digital inputs available:
< Analog Input 1 & 2 and S/PDIF input
< Analog Output 1 & 2 (available on both TRS and unbalanced Outputs 1 and 2)
< S/PDIF Output (mirrored at unbalanced Outputs 3 and 4)
At 24-bit operation, the Fast Track Pro operates as a two-in, four-out device or a four-in, two-out device at a maximum sample rate of 48kHz, with analog OR digital inputs available:
< Analog In 1 and 2 or S/PDIF
< Analog Out 1 & 2 (available on both TRS and unbalanced Outputs 1 and 2)
< S/PDIF Out (mirrored at unbalanced Outputs 3 and 4)
Or
< Analog in 1 and 2
< S/PDIF In 1and 2
< Outputs 1 and 2 or 3 and 4"
 
Generally Sonar is going to read available outputs from the driver for the interface, so if your interface is set to use 4 in and 4 out simultaneously, then 24 bit will not be available in the pod driver setup and hence not in Sonar.

Hi. Just popped in to say thanks to slartabartfast. I did not know that about the Pod It should (hopefully) help OP as well.
All the best.