I have a new PC speakers, creative T40, and realizes how terrible my TV HiFi is. I would like to upgrade my HiFi and I dun really know where is the problem.

My HiFi system was used for 7-8 years and simple one. Two Speakers (Energy C-50C) connected to analog amplifier (Lzban AV-02). Analog output from TV's 3.5mm jack with RCA converter (and I just know that TV has an RCA output). The analog cable is shielded tho maybe has damage on shielding or copper oxidized. TV is sony KDL-42W650a. Not buying a new TV, TV output only have 3.5mm jack and RCA. Have a blueray player with coaxial ouput.

A few stupid questions would like to ask:
1) I want to use the TV hifi for spotify. Two ways of sources I can think of: either RCA output from a new pc to amplifier or USB android plugging into PC. Which is better?

2) Is it worth to replace my cheap amplifier to a new analog amplifier? If yes, any suggestion?

3) I am open to DAC or optical or coaxial amplifier if qualified RCA converter is available on market (not more expensive than buying a new TV with digital output)

4) Is it necessary to replace my old speaker?

I suppose all decisions are related to costs which I can afford are very limited...not more than $400

I hope all are clearly described and some experts can help on this!