other than the issue of mono(geetar signal) and stereo(our music), another thing is the voicing of the amp.
Geetar amp and bass amp have their own frequency range which are most suitable for the instrument being plugged in. For the music we are hearing from cd and such, it consist of a wide range of frequency which might be out of the range of the instrument signal frequency range(which work within the vocing of the amp itself). Thus, even with the music able to connect to guitar amp/bass amp and broadcast out, the sound will either be too bright/dull(thru guitar amp) or boxy sounding(closed back bass amp). Both which i have tried thru my own setup and noticed the differences.
Of course there wont be any problem if just wanting music being played thru the amp and a tweak of the eq section to boost/cut certain frequency of the music signal to make it sound better.
Right or wrong issue aside, if just wanna sound being heard, connect away. If wanna right tool for the right usage, then use it the right way in the right method
another thing bout converting stereo to mono, if wanna plug cd player/mp3 player/radio receiver to guitar amp/bass amp, one easy way is to get 1/8' plug stereo cable from home fix or any audio related shop.
Chop of one end, expose the wire, you will see 2 insulated wire and a unshielded wire. Twist the unshielded wire together, this will be the ground wire. The 2 insulated wires carry the left and right signal of the music we hear in stereo. Twist and combined these 2 wires together, this will be the signal wire. With the ground and signal wire, solder it to a 1/4' mono phone plug.
once done, connect the 1/8' side to our music player earphone jack and the 1/4 ' side to instrument amp input and you will hear the music thru the amp, all in mono. What ever stero effect, panning etc, will be lose. Heh, somehow like listening to music on BBC..
if having too much time on hand and bored out of life, anyone can do it...