Shoving it on ebay isn't going to help you at all, just going to end up spending more money and still end up in the same situation.
If the amp sounds fine by itself, then you've got the wrong pedal, not the wrong amp. Anyway, muddiness is caused by too much bass prior to distortion, so it could even be the guitar's fault depending on how your pickups are set up and what you use.
Also, are you actually sat directly facing the front of the speaker? That's your sound, stood over, behind, to the side, in a different room etc isn't how it sounds. It's quite easy to think you sound muddy when in fact you don't. Why do you think mic'ing up an amp often sounds nicer off-axis?