I'm going to assume that the gear is not out of spec and doesn't need repair and there's nothing the manufacturer can do to correct this issue short of selling you a different amplifier.
I don't know what consumer laws are like in Norway or Sweden. But I'm going to guess that at some level they're not that different from the United States or Canada. I'll also say that I'm not an expert in consumer rights.
If a brewery advertises that a beverage "tastes great, and is less filling", what really can you do if it doesn't taste great and isn't less filling? Presumably it tastes great and is less filling to someone, all they really needed to do was to get one person to attest that this is true and away they go with an advertising campaign.
Most of these claims are often subjective claims. Is the amplifier dead silent? It was to someone. Does this solid state amplifier sound like a pair of SET monoblocks? Yes, it does to someone. Maybe that someone isn't you, but I'm sure there's someone in the world who would say "Yes, it sounds the same as SET to me." Just go to Audioholics and you'll see plenty of posts that claim there is no difference between amplifiers, that jitter is inaudible in modern electronics, and that HT sounds just as good as dedicated 2-channel.
If they advertised a certain S/N ratio, and you're not getting that S/N ratio, maybe you're onto something. But how are they measuring that S/N ratio and under what conditions did they achieve that? My guess is that they can back up the S/N numbers with measurements, just like lots of audio manufacturers that sell to big box stores advertise gigantic power numbers for their amplifiers.
But let's assume that their absolute measurements were wrong for a moment and that you've got something here.
In 2001, my friend received a letter from Mazda about his late model Miata. Mazda had advertised the Miata as having a certain horsepower. After the cars were already sold, they found that they had over-estimated the HP, and it was 5 less HP than advertised. If I remember correctly, what they did was offer to buy back the car at fair market value (not the new price) or send a cheque to my friend for $1K. But this offer wasn't good forever; you had to make a decision.
Was the amplifier substantially different than was claimed? For the areas which you find that it is substantially different, my guess is that those things are subjective (depends on the person), contextual (depends on associated gear and environment) and not absolute.
Even if we could all agree that things aren't as they were originally claimed, hey, it's been 3 years. My, that's a long time. A reasonable person would conclude that after 3 years of inaction and ambivalence that you had accepted the amplifier as good and appropriate and that you were satisfied with its performance.
If the manufacturer is offering you trade-in value for your amplifier, then I think they've gone above and beyond the call of duty. If the manufacturer is simply offering a small, marginal discount then sure, they could be doing more, but they don't really have to do anything.
Perhaps they're really trying hard to do the right thing, but it would be unreasonable to expect a 100% refund on gear that was sold to you 3 years ago. If the company had a return policy at the time (a number of audio manufacturers offer 30 days), you should have returned the gear way back then.
