I have a Vox AC30 with stock Wharfdales. I just purchased a Weber Alnico Blue Dog 8ohm 15 watt speaker for it. I'm still deciding what the 2nd speaker will be and I know it has to be 8ohms but does it also have to be 15 watts or can it be 30 watts?
The way it was described to me is the wattage rating of the speaker only describes how much power the speaker will handle before it starts to "break up" (I still am not sure what that sounds like). Supposedly, that rating is for continuous power, not transient peak, which could be double. I think I remember getting this little "lesson" in an email from Weber several years ago, so other brands may be rated slightly differently. Generally, a 30 watt Weber will handle 60 watt transient peaks, and Weber says theirs are rated conservatively.
So if you are wanting to overdrive the lower watt speakers and get "break up" then the 30 watt will break up later than the 15 watt will. And remember that the total watt rating of the cab will only be twice the lowest rated speaker......or in your case, 30 watts......assuming you only use two speakers.
This is confusing to a lot of people, because there are a lot of terms used that aren't normally explained well enough for people to make educated decisions about speakers. There are a lot of very knowledgable people on TGP that can help you learn to decipher it.
The second speaker can be anything over 15 watts. When you wire two speakers together, they most they can handle as a pair is double the lowest wattage speaker. A 15 watt and a 30 watt speaker wired together can only handle 30 watts total, not 45 watts.
Speaker wattage is actually a heat rating of how much heat the voice coil can take before potential meltdown. When you get into specifics of speakers, there are a lot of different things that will apply as to how well the speakers preform.
Efficiency rating is another commonly looked at term that you should also take into consideration. When running a pair of speakers, most people prefer having two speakers with similar efficiency ratings. This also has to do with how loud the perceived sound is. A difference of 3dB is an enormous and noticeable difference. If you have one speaker rated at 98dB and one at 101dB you will mostly hear the 101dB speaker.
There is a lot to this, but that is a good start. If you want more in depth info, just ask.
AC30 = 36 watts before distortion.
Alnico Blue = Claimed Wattage 15 watts each
Second Grade Math = 2x15 = 30
Third Grade Math 36/30 should = Poof
That whole thing wasn't really as amusing as I had hoped. A quad of EL84s pushes close to 36 watts, pre distortion. An AC30 at full tilt is closer to 50 watts. (Can't remember the exact number, but I can measure it if you really want to know).
For whatever reason, the Celestion Alnico Blue's stated wattage rating is much less than it can actually take.