So, that is just for the amp section and it is 25kg which makes it 55lbs and the outboard power supply that comes with it is 35lbs. Not that weight along is an indication of excellence. But, note that it ships standard with the power supply in a separate box to ensure the amplifier section is as clean sounding as possible.
Naim is quite refined in terms of it sound and presentation. The 500 series is designed to be paired with the Focal Utopia line of speakers and the folks I know who have this amp (I know a few) feel it is fairly priced.
One other thing to note, it may be 140w into 8 ohms, but this amp is not "stable" at 2 ohms where it can deliver two ohms for a few seconds. This amp can drive 2 ohm speakers. There are few amps that can deliver the kind of current that Naim is capable of over extended periods of time. Match it with the hardest to drive, lowest impedance speakers you can find and this amp with thrive.
For many, what Naim does is not for them and is not really required. If you have really hard to drive speakers, Naim should be at the top of the list. In an extreme example, I had an old pair of Infinity Kappa 8.1s. I would not buy a $35K amp to pair with 25yo pair of $1700 speakers. But a Supernait is in-scope. Those speakers had a min impedance of 1.7 ohms and the Supernait is one of the few integrates that can handle them without constantly blowing fuses.
Full disclosure. I am a Naim dealer for the Uniti and Classic lines. I do not have access to the 500s but have heard them on several occasions.