Likely culprit for harshness at higher volumes?

Saru,

It is obviously not your choice of music. I doubt it is the amp or preamp.

Therefore that leaves speakers or a jittery digital input.

One further possibility is your room, if you have tile on the floor and fairly reflective surfaces then this could be the issue - an issue that gets worse as you increase the volume.

I'm going to go out on a limb and say that it's probably your speakers or the room. The cables you describe should work just fine. I recently compared a set of $2,000 Nordost cables to a $7 pair of hardware store cables and certainly couldn't tell $1,993 worth of difference, at least for my system that I have about $5,000 in for the main stereo components. Cables make small difference and what you're describing sound like they are more significant in scale. Keep in mind that there are many members on the forum that have systems where cables might make a significant sound difference, but I don't think it applies to you and me. I'd suggest going with something like Blue Jean cables if you want something that's respected and affordable.

I would try decent (not necessarily expensive) analog cables instead of the Toslink first.

It's your speakers.

From a review: "The overall sound can get a little grainy if pushed too hard"

perhaps clipping.
Source music is awful, as has been noted by Shad, above.......look up 'loudness wars' if your want more info.

Also, Since speakers so not have watts, I'd simply ignore that rating. Good, loud, dynamic music demands a lot of an amp. Maybe it's just not passing muster.