Why does Computers - Amp - Speakers sound better?


I have a budget system. Denon turntable, Shure cartridge, NAD integrated amp, and Wharfedale speakers. For some reason, computer -> amp -> speakers sounds SO MUCH BETTER than turntable/cartridge -> amp -> speakers, even when the source file on the computer is 256 kbps mp3.

I don't know why this is. It would make sense if my amp and speakers were leagues above my turntable or if the source file was in 320 or FLAC, but that's not the case. 256 kbps should not sound better than high quality vinyl of music from the past five years (i.e. not mastered from bad tapes).

Is my turntable or my cartridge (or both) inferior to my other equipment? Also, I'm using Monoprice cables for my computer ---> amp and off-brand cables for turntable ---> amp. Could that be why?

thank you. The reason why this bothers me is because I spent over 1000$ on my vinyl (not just for the better sound, but because I'm deeply involved with the new-wave punk scene) and $400 on my turntable/cartridge and it bothers me that a low-quality file produces better sound.
toxicwaterfront
If you aren't "level matching" between sources, studies say the louder of two things being compared will be thought to sound better.
My first thought is you need a good phono pre-amp, one better than the internal phono stage in the NAD, but I'm not a vinyl guy. I agree, 256K rips shouldn't sound better than your vinyl rig. How are you getting the signal from the computer to the amp? Are you comparing vinyl and 256K verions of the same song?
Some basic questions:

1)Is the cartridge new, or otherwise known to be in excellent condition?

2)Are you sure that the cartridge is optimally installed and set up, in terms of:

(a)Overhang
(b)Offset angle
(c)Tracking force (preferably near the upper end of the range recommended by Shure).
(d)Anti-skating force (if the turntable has a calibrated anti-skating dial, and its numerical value is set equal to the tracking force, it is most likely much too high).
(e)VTA/SRA (if adjustable)
(f)Azimuth (if adjustable)
(g)Capacitive loading
(h)Resistive loading

Also, try to describe more specifically the ways in which your records don't sound good.

Regards,
-- Al
Have you tried recording your vinyl to digital and comparing the sound to your 256k files. Exactly how is it better?
You are making a lot of assumptions. "256 kbps should not sound better than high quality vinyl of music from the past five years (i.e. not mastered from bad tapes)." Why not? How do you know the tapes are bad? To get anywhere, you are going to have to listen to the same recording on both sources. That will at least give you a frame of reference to start with. Just to clarify, get the same album on CD and record. Also, make sure that one is not a remaster and the other one is. They have to be the same. Also, when you import the CD, try it different ways (lossy, non lossy, FLAC, WAV, etc). Doing it this way makes it much easier to focus on the equipment.

For me personally, I can tell you that I prefer older vinyl recordings that have no digital in the chain. Newer recordings or digital remasters sound like better digital and not like analog. I don't think its worth the extra time and money. Keep in mind, thats just my opinion.