I have this 80's Champ 12, red knob model with a 12" speaker. I currently have a Celestian Vintage 30 in it. The amp has little or no bass responce and is very middy. Can a speaker change help, or is it a flaw of this circuit? What would be a good speaker choice?

Given the fact that you want more bass and less mids you could check the following speakers:

Celestion G 75, G 30 H or G 100
Jensen Neodym or Tornado

Maybe also one of the Eminences would work.

Have you tried the amp through an extension cabinet which is known for good bass response? If the bass is still not enough with that cab then we can look to the amp for mods.