If you have really old speakers you should probably start with replacing them. Some people puts most of their stereo budget in speakers and if you follow this you could look for speakers from $400 up to around $1000.
If you want to upgrade you digital part I have two ideas. You could get the Audioquest Beetle and a Chromecast. That's about $250 and should improve the sound. Or you could sell your amp and get a PS Audio Sprout, it has everything except speakers. If you like to tinker you could build a Raspberry Pi with a soundcard or spdif outputs and connect to your amp.
If you want to upgrade you digital part I have two ideas. You could get the Audioquest Beetle and a Chromecast. That's about $250 and should improve the sound. Or you could sell your amp and get a PS Audio Sprout, it has everything except speakers. If you like to tinker you could build a Raspberry Pi with a soundcard or spdif outputs and connect to your amp.