I think you’re right but it’s just more powerful capable of driving more demanding headphones but it’s a NAD so I wouldn’t be surprised if any of the amps can’t drive beefy cans anyway
They don’t really go into specific detail on what’s different though.
The 3020 goes loud and low and at -20dB it will be damaging to ears or headphones if used for too long.
I've always likened the sound to representing the output from the speaker terminals, pleasing and I guess you could call it faithful.
There's a lot to like about the Bostons. As long as the impedance plot isn't too much of a roller-coaster then they'll make the most of the 30W on offer from the little class D amp in the D3020. Whether you like the sound though...
The Cambridge Audio SX50 are a little more squat. They're also an 8 Ohm 90dB speaker too. High efficiency is pretty rare in budget bookshelf speakers. Most are around 85-87dB. You're normally looking at £250+ for 90dB. The other slightly worrying trend is the increasing numbers of 6 Ohm speakers.
In the end I think you need to sit down and have a listen. Arrange a demo and take your amp along.