Tony Pasko the GEAR411 guru at The Rock House Method sent an email the other day telling me that he has an article in the current issue of Premier Gutar Magazine on speaker tone.
In looking at the article Tonys background and knowledge about speakers come shining through, "The thing you want to consider is that what you like or dislike about your guitar tone could very well be the speaker—it is after all 40 percent of your tone. Picture your amplifier as a sports car. Now picture your speakers as the tires. The wrong speaker in an amp is like having bicycle wheels on your sports car. That’s how important the speakers are".
He goes on to talk about Wattage vs.Volume with some simple to understand charts.
Check his article out at the link below, it's pretty good information
Speakers: 40% of your tone