Compression it is, but not from the master compressor which does exist, but rather the samples themselves are compressed as both audio and for size purposes, as files. That kind of compression.

But the studio-sound is not always a bad thing.

I don't know how all the drums kits are with PSR 770, but to my ears especially bass drum and snares, but hihats too sound dark, close, liniar, with no detail, no vibrancy to it. Hard to describe. I guess that's why there is Genos now - which sounds very different that what Yamaha was before it.

Anyway, it is what it is. Next models probably will take quite a lot from Genos, as competition is forcing them to update their sounds. Korg I mean.

And who knows, maybe Roland (one way or another) will have yet another comeback, after the BK-9 and will surprise us in a nice way.

But if at least there was the option to load-up new samples, the drum sounds could be helped. And there is, it seems, people have done it and the results, like the link I posted above, are nice. So, to answer my own question, yes it is possible.