Affordable Processor with Fabulous 2 Channel ?

@freemand

The Classe Sigma SSP is optimized for stereo or when used as a stereo preamp and its DAC is very good especially when fed via its USB input. Its USB input by far sounded way better than its other inputs for digital music files. I will guarantee that the CD disc played back through your Oppo 103 connected to the Sigma SSP via spdif (coax digital) or HDMI sonically cannot compare to those same CD properly ripped in digital files (FLAC) and played back through your computer feeding the Classe Sigma SSP via USB. The latter by far will be superior sounding.

Do not hesitate to purchase the Classe Sigma SSP. It also performs and sounds really good when used as an analog stereo preamp in bypass mode. If you have a high end CD player or DAC and want to feed the Classe via analog and use the Classe Sigma SSP as analog stereo preamp you will have to select digital bypass mode thus bypassing DSP and other digital processings in the Sigma SSP so the analog signal will be kept in pure analog domain all the way through its analog output stage or otherwise DSP will get in the way and will degrade sound quality.

The Classe Sigma SSP is your best solution for combining your stereo and multi-channel needs in one single box at a very attractive price on a used unit.
Its surround performance for home theater is also very good. You probably won’t need the NAD M51 and just use the DAC in the Classe. The NAD M51 was great and is still good by today standard but the M51 is a bit older measured by DAC or digital audio technology and the Sigma SSP is newer and uses newer and up-to-date DSP and DAC or digital audio technology.

You can try connecting your NAD M51 DAC to the Classe Sigma SSP but first you will have to set the output of your NAD M51 to fixed and set the Classe Sigma SSP in digital bypass mode and make comparison between using the NAD M51 to the Classe via analog with the Classe set in bypass mode vs streaming digital files directly to the Classe via USB input on the Classe.
That way you will know and find out which will sound better to your ears.

Caphill,
You sure do know the Classé very well. I have it and absolutely love it.  
  Help me understand why the Sigma would perform better with a connection via USB compared with a HDMI connection?  Wouldn’t  these be run through the same DAC’s?

Yes, I would like to know why you think USB is better than digital COAX for PCM audio.  Obviously, USB is required for any DSD files, but I think the compromises made in the USB interface may get in the way somewhat when compared to COAX s/pdif with a good digital cable and transport.

@bwguy

I did own the SSP 800 for over 7 years until recently upgraded to the Lyngdorf MP-50 but am still using all Classe Delta series class AB amplifiers in my dedicated home theater room. IMO the Lyngdorf MP-50 is better than the SSP 800 in all areas when used for surrounds (home theater) with Room Perfect properly set up and calibrated. Hands down. Plus I wanted Dolby Atmos & DTS-X. The SSP 800 is outdated and is time to move on.

Now the Sigma SSP is little different. It’s optimized for stereo but its surround performance is still pretty good but its stereo performance for music that stands out. That's the area where the Sigma SSP shines. IMO if it’s only for surround (HT) purposes I think there are better and cheaper options out there than the Sigma SSP. IMO the Anthem AVM60 with its ARC2 outperforms the Sigma SSP for surrounds. And I think the Marantz AV8805/AV8802 are better choices than the Sigma SSP when used for surrounds and will likely outperform the Sigma SSP. The Sigma SSP is good for someone who wants to combine his stereo and multi-channel needs in one single box solution. Its stereo performance for music playbacks are phenomenal coming from an AV pre pro especially for its asking price of $5k and $6k new. The original version of the Sigma SSP nowaday can be had for around $2500 in used markets.
But if it’s for a dedicated home theater setup strictly used for surround movies I wouldn’t get the Sigma SSP. The XLR are limited only for front L/R channels with the remaining channels are only available in single-ended. It lacks room correction.

@auxinput

The USB, spdif, Toslink as well as stereo analog audio input design stages and its signal path implementation & configuration on the Classe Sigma SSP were pretty much copied and pasted directly from the Classe CP 800 stereo preamp/DAC.
If you google Classe CP 800 review and click on any reviews for the Classe CP 800 there are articles that will explain why its USB input sounded better than its other inputs. That way I don’t have to repeat what they said. There’s also a forum thread on Audioficiando I believed that confirmed the findings. I myself also found the difference in sound quality between using their USB input vs other inputs and the difference wasn’t subtle assuming you use a very good quality USB cable. The CP 800 stereo preamp/DAC is built around its USB input and is optimized for use with its USB input with the rest kinda after thought design.
The CP 800 isn’t for someone who’s looking solely for a solid analog stereo preamp. I would go with something else if I want an analog stereo preamp as there are better options out there in a similar price range as the CP 800 ($6k new retail price).

@auxinput @bwguy

Here’s a link to one forum thread re the finding that proved the USB input on the Classe CP 800 stereo preamp/DAC is superior sounding than its other inputs and this also applies to the Sigma SSP as the USB circuit design of the Sigma SSP is lifted directly from the CP 800 and both the Sigma SSP and the CP 800 use same DAC chip sets but the CP 800 uses 2 DAC chips per channel whereas the Sigma SSP uses only 1 DAC chip per channel for its front L/R channels.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.audioaficionado.org/show...