Picking an amplifier

Yeah, you would think that COAX and optical should sound exactly the same.  Based on electrical engineer point of view, it should be the same and it's just data.  But from an audiophile listening standpoint, the COAX will just sound better.  The optical can get very good if you use a high end glass fiber cable such as Lifatec or Wire World Supernova, but the COAX will just sound better in the end.  The COAX also has a more natural ambience to the sound.  You can read up on it if you wish.

The Anthem 5/50 amps are nice choices.  The MCA 50 has a power supply that is 50% larger than the MCA 5.  The MCA-50 has 150,000uf capacitance, where the MCA-5 has 100,000uf.  This means the MCA-50 may be a more refined sound and it will also have a bit more muscle for the bass/midbass (even at lower volumes!).  Obviously, the MCA-50 costs more.  The Anthem amps will be a lot cleaner/faster/detailed sounding than the NAD, and probably more power/muscle in the bass.

Okay, I remember the reasoning behind why the PCI version of the Xonar Essence card (ST version) was better.  It had to do with the PCI bus clock speed, which is 33Mhz.  For audio data, you really don't need a super fast bus.  The PCIE bus clock speed is in the gigaherz range (2.5Ghz / 5.0Ghz / 8.0Ghz).  I think this created much more problems for jitter in the data.  The 33Mhz PCI interface just worked better and was more forgiving for sending audio bitstream data.

Ah you posted almost at the same time.  A while back I did some research on toslink cables.  The Audioquest diamond is extremely expensive.  I don't know if it is really worth it.  lol.  Looking at the other top toslink cables I found:

Lifatec - 470 Audio Data Silflex Glass Fiber Optic strands polished ends.  3 foot cable $81

Wire World Supernova 7 - 338 strands Borosilicate Glass polished ends.  1 meter cable $200.

Audioquest Diamond - 280 Narrow-Aperture Quartz (Fused-Silica) Fibers polished ends.  0.75 meter cable $499.

From my research, the Lifatec was the absolute best cable for the money, and probably compared to higher priced cables.  Lifatec makes fiberoptic cables for medical grade devices.  The Wire World was the only other one I would have considered as the Audioquest is just insanely expensive.

Wow, that last link is some interesting stuff!  I'm glad to have my statements substantiated.  A couple quotes from that page that are interesting:

"the TOSLINK interface of red led is not fast enough"

"That square directly from Toslink receiver is bad bad bad. It is a miracle the DAC reads and understands this crap."

"TWEAKING OF A CD PLAYER to make it a killer transport...Changing the sockets from RCA to BNC or much better to XLR on both the transport and the DAC while changing the resistor from 75 Ohms to 110. "

I have also read feedback that BNC coax is highly superior to RCA coax.  That's a given.  However, many of our equipment is limited by RCA only connections. lol.  I have also read from a posting that the XLR AES/ABU interface is still not as good as a proper BNC COAX.  That was based on listening tests and well as some technical justification.  I can't remember what exactly they said.

It's proven that there are differences in digital COAX cable.  The solid-core silver or silver-plated cables are highly superior because the silver is able to charge/discharge much faster than just copper and it allows better accuracy in the extremely high speed digital square waveforms.  One cable I have heard is excellent is the DH Labs Silver Sonic D-750, but at $200 or so for a 2-meter cable, it can be above your budget.  I always recommend the Blue Jean Cable Beldon 1694A because it's an excellent digital cable for the money (much better than others if you don't want to spend much).  It's documented that 1.5 meter is the minimum/best length because shorter cables will cause signal reflections back to the transmitter and distort the waveforms.  I generally recommend 6 feet (which I use).