I have a Peavey vypyr 75 watt and the tone that comes out of the speaker is okay I guess. It works I guess but it is nothing to really go on about, but the tone that I get from the headphones is just absolutely amazing!! I was very surprised at the tone I was getting. So I was wondering if the problem of why I don't care for the speakers tone and love the headphones tone was the speaker itself and if so what could I replace it with to better hear what I'm getting through the headphones instead of what I'm currently hearing through the speaker. The speaker that is currently in it is an 8 ohm peavey blue marvel which I've heard aren't that great if that helps.
It is quite possible that you could get a better tone. the problem with speaker swapping is that you don't know what it will sound like until after you've paid your money and tried it.

If your amp has an output for an extension speaker then you could try it with other cabs for free, so long as you have mates or a friendly guitar shop who will let you try.

Also try the Eminence and Celestion websites which have sound clips of their speakers to help decide the tone you are looking for.

Not all speakers will work equally well in your cab. Once you have decided what to go for I should be able to tell you if it is an absolute no-no. I'm a bassist so other people will have to advise you on tone.