Integrated Amplifier for Monitor Audio Silver S2 Speakers

Get yourself a Parasound Halo Integrated.  It has plenty of power, has a great built in dac, has tone controls, 160wpc, phono input, balanced inputs and outputs and sounds wonderful.  I replaced a Rogue Cronus Magnum Integrated with this and couldn't be happier.

Hi djengan, 
I'm in agreement with Viridian,  amplifiers do make a significant difference.  You've already proven this fact with the Accuphase amplifier placed in your system,  clearly superior sound to the NAD.  Electronics make a considerable impact.  It isn't just about the speakers,  yes they're important but tend to be over emphasized in my opinion. I've heard numerous examples where upgrading an amplifier resulted in significant sound quality. Can speakers do this? Of course,  but so can power amplifiers. 
Best Wishes, 
Charles 

Djengan,
Roxy54 who posted above has very good ears and taste IMO. The Sugden A21 SE he recommended is worthy of more scrutiny.  A solid state pure class A amplifier noted for really fine sound quality  (very organic and sophisticated). Built with exceptional parts quality. 
Charles 

Everyone has to do things their own way. However, some of the other posters recommend formulas for how much your speakers should cost relative to other components. There's really no nice way to say this, but if you select components that way, you're asking for a train wreck. The only thing you can be certain of when you spend more for a product, is that it costs more. There's absolutely no reason why a pair of $1000 pair of speakers driven by a $2000 integrated amp, can't sound better than $2000 speakers with a $1000 amp. There's so many reason's for this, I couldn't even scratch the surface going over them in a post like this. But I can give you a couple of examples. 

When you look at a speaker, where is the money going? Companies like Vandersteen and Magnepan put very little into the outward appearance of their speakers, where companies like B&W and Wilson do. The ratio of performance to appearance is very different. If you're going just by price, you can easily pick a more expensive speaker that isn't as good as what you already have. 

Selecting components is all about system matching. How well does a component work with your other components, and is the change positive or negative? The better you get at matching components, the less money you need to spend. 

You don't have to take my word for any of this. Do some listening and you'll see this for yourself.

After looking at both the subjective parts and the measurement-based parts of John Atkinson's review of your speakers, and after looking at descriptions and specs of the NAD C356BEE (although I'm not sure if that is the version of the C356 you are using), my suspicion is that BOTH the amp and the speakers are contributing to the lack of detail you described.  With the speakers probably contributing somewhat more significantly.  For example, from the review of the speakers:

The most obvious characteristic of the Silver S2's sonic signature was its mellow treble....
...the speaker does have an identifiable character, being balanced on the warm, mellow side, this is probably a good thing, given inexpensive electronics' tendency to have glary trebles....
... The low-frequency -6dB point coincides with the port tuning frequency of 42Hz, but a slight energy excess in the upper bass is visible. Together with the overall depressed level of the tweeter, this will give rise to the warm, rather mellow balance I heard.

All of these characteristics are consistent with compromised presentation of detail.

Before proceeding any further, here are some things that occur to me that might be worth trying if you already haven't:

1)Position the speakers so that the tweeters are aimed directly at the listening position, and are at or close to ear level.

2)Turn off the amp's soft clipping feature, if you have been using it.  (Although when you do that be careful not to turn up the volume high enough to cause clipping distortion, which would be obvious on dynamic peaks in the music).

3)Activate the amp's tone defeat function, to take the bass and treble controls out of the signal path.  (Tone controls can often have adverse effects on transparency, even when set to their neutral position.  Especially in relatively inexpensive designs).

4)But also try turning off the tone defeat function, and using the amp's tone controls to either boost the treble a bit, or reduce the bass a bit, or both.

Good luck.  Regards,
-- Al