I've not read any "real world" users give an A/B comparison of the Caiman and Rega DACs. Nor have I read any comments from the developers of the Rega DAC which give their views on USB audio. Rega tend to follow their own path and may have had a particular market in mind and would also be aware that USB audio is a bit of a moving target. Hence there is only a basic 16bit 32-48KHz USB input on their DAC. Mind you, neither does MF's latest DAC nor the SimAudio 100D or the SimAudio 300D have hi-res usb. It is interesting to read what Sim Audio have to say about USB audio on thier website,
I've quoted it here for convience (note to mods: please remove if I broken a rule):
An important note about the USB input
The USB input on the MOON 100D accepts a digital audio signal of up to 16-bits/48kHz. This is the native
specification of the USB input. However, if you require a higher sampling rate, there are ways to achieve
this with the 100D.
Rather than including a USB input for sampling rates exceeeding 48kHz, we opted to provide the best possible
power supply, D/A converter and analog stage in the MOON 100D at this price level. Also, keep in mind
that USB wasn't originally intended for use with hi-resolution audio; there are latency issues which compromise
sound quality (i.e. jitter and phase errors, etc.). These could be minimized through dedicated costly circuitry,
but this would significantly drive up the cost of the 100D without improving the sonic performance on any
of the other inputs.
If you intend to use the MOON 100D's USB input, it's very likely that your computer is a data source for music.
Today most computers with a built-in sound card will have either an S/PDIF (RCA connector) or Toslink
(optical connector) digital audio output, thus eliminating the need for a USB input on the 100D.
Furthermore, if you're committed to getting the most out of the music stored on your computer, then in all
likelihood you have installed an after-market soundcard in your computer. Virtually all of these soundcards
have at least one S/PDIF, Optical or AES/EBU (XLR connector) digital output. At this point, USB connectivity
becomes irrelevant. The USB input is there for convenience as opposed to performance.
In the event that you wish to use your computer's USB connection with files greater than 48kHz, there are
several relatively inexpensive after-market products; compact one-box USB-to-S/PDIF interfaces that will
work with USB datastreams up to 24-bit, all the way up to 192kHz.
Of course, others trumpet asynch USB as the answer to evrything and that hi-rez is a must have. Perhaps the Rega DAC plus a MF V-link is the answer, but we'll have to wait a while for a decent level of feedback on just how good or otherwise the MF USB-to-S/PDIF converter actually is. Or it's a case of waiting for the Audiolab 8200DQ. In any case, you certainly have the sort of kit that could benefit from a DAC upgrade. It's all down to what kind of money you have to invest in a new device and auditioning all the alternatives.