I cannot understand how this particular audio chain can suffer such quality loss. Everything is digital until it hits the CDP, and I don't think there's a fault within its course.
Well, you’d be absolutely wrong. I’m guessing you bought a cheap streamer and hooked it up to a computer that’s used for things other than audio. If that’s the case it’s no wonder your father, who’s obviously more attuned to high-end audio, is less than thrilled with what he’s hearing. The computer and streamer are riddled with cheap parts, noise, and timing errors. It’s not as simple as just 1s and 0s, and everything matters in the audio chain. BTW, are you using ripped CDs and/or a subscription streaming service? That’s a whole other can of worms.
Given the apparent quality of your father’s system and the level of sound he’s accustomed to, if you want him to really enjoy the streaming experience I’d recommend ponying up for something like an Innuos Zen Mk3 streamer/server that is purpose built to deliver high-end audio that will more impress and satisfy an audiophile like your father. Yes it costs $2600, but what’s the point of spending less if your father doesn’t like it and likely won’t listen to it? Just my $0.02 FWIW. Best of luck.