Oops, must have been thinking about something else. 

In case anyone's interested, here is an M51 in excellent condition.
http://www.head-fi.org/t/734087/nad-m51

Has anyone tried to reverse FW from 1.43 to 1.39? 
Mine is 1.41 from the factory and updated to 1.43 but wanna try the 1.39 version.

ellevoid said:

  Has anyone tried to reverse FW from 1.43 to 1.39? 
Mine is 1.41 from the factory and updated to 1.43 but wanna try the 1.39 version.

Click to expand...

you need to roll back to 1.41 first, then 1.39 or else the M51 won’t work.

lcfiner said:

 
you need to roll back to 1.41 first, then 1.39 or else the M51 won’t work.

Click to expand...

Downgraded successfully. Thank you so much
 
very first impression is like everyone said 1.39 has a little fatter bass and more kick, but 1.43 seem to sound less congested/a little better separation.
 
So for my system (HD800, HE6), I think 1.39 is suits me better.

How this NAD M51 vs Audio GD M7?

I never heard of Audio GD M7, but their products I tried they all have kind of metallic sound.
 
I found that when I connected NAD M51 to two different amps. One using XLR out and another one use RCA out.
The RCA out will sound totally suck, bass is gone, high is gone, no dynamic at all.
 
Is that normal for NAD M51?

ellevoid said:

  I never heard of Audio GD M7, but their products I tried they all have kind of metallic sound.
 
I found that when I connected NAD M51 to two different amps. One using XLR out and another one use RCA out.
The RCA out will sound totally suck, bass is gone, high is gone, no dynamic at all.
 
Is that normal for NAD M51?

Click to expand...

No. Probably more of a reflection on the amps than the M51. I've used it into both types and found its sound consistent.

kmanchu said:

  No. Probably more of a reflection on the amps than the M51. I've used it into both types and found its sound consistent.

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And can you confirm the sound differences between the inputs that some refer to? Are they still there with updated software? Thanks.

unsleepable said:

 
And can you confirm the sound differences between the inputs that some refer to? Are they still there with updated software? Thanks.

Click to expand...

In my personal experience, I still run the original firmware on mine and never updated it because I felt no need. The accounts I from NAD directly was that the firmware changes only ever reduced the volume output by 1dB to prevent digital clipping highly compressed recordings. I honestly haven't looked for improved USB performance by chaning the firmware. That said, I think the inputs are all good, but to me a VERY good SPDIF converter gives better performance over the built in USB. Also, a USB power conditioner like the Schiit Wyred or iFi iUSB Power helps the USB input also. I think that the AES input is the best, this is probably because it is transformer coupled and providers better electrical isolation from the source. But the coax input is also very good when used with something like an Audiophilleo. Don't get me wrong, the USB input is good, especially with a power conditioner. But the addition of a good SPDIF converter, IMHO, takes the performance level to another league.
 

ellevoid said:

  I never heard of Audio GD M7, but their products I tried they all have kind of metallic sound.
 
I found that when I connected NAD M51 to two different amps. One using XLR out and another one use RCA out.
The RCA out will sound totally suck, bass is gone, high is gone, no dynamic at all.
 
Is that normal for NAD M51?

Click to expand...

One reason XLRs can sound "better" is that the level (voltage swing) is much higher balanced than single ended. So if you are not volume matching then the simple higher volume will sound like it has more impact, bigger, etc. All you are really getting is the volume turned up.

LOL i tried to roll back frmware on my nad M51 . mine is a newer unit and has version 1.43 
i rolled back to 1.41 and it would not work , had to update it  again back to 1.43 .
i thought for  a while its going back to nad but it allowed me to go back to 1.43.

too bad the remote is not backlit
Q : when i got it on fixed vol it locks at -20 , any way to lock it at -10 or something around that?

ellevoid said:

  I never heard of Audio GD M7, but their products I tried they all have kind of metallic sound.
 
I found that when I connected NAD M51 to two different amps. One using XLR out and another one use RCA out.
The RCA out will sound totally suck, bass is gone, high is gone, no dynamic at all.
 
Is that normal for NAD M51?

Click to expand...

Could you tell me which Audio GDs you tried that have metallic sound ? 

ellevoid said:

  I found that when I connected NAD M51 to two different amps. One using XLR out and another one use RCA out.
The RCA out will sound totally suck, bass is gone, high is gone, no dynamic at all.
 
Is that normal for NAD M51?

Click to expand...

 
I recall that happening to me when I tried connecting both the XLRs and the RCAs at the same time (to compare 2 different preamps). Should sound fine if you have only one pair connected.

jdoz said:

   
I recall that happening to me when I tried connecting both the XLRs and the RCAs at the same time (to compare 2 different preamps). Should sound fine if you have only one pair connected.

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Yes it sound great with just one paired connected. It just I got 2 amps one is solid-state another one is tube and I don't want to reconnect the ICs every time I want to switch between the amp.