A Question on Speaker Driver Efficiency


I have been tweaking my guitar amps, by upgrading the speakers.

I installed a larger speaker (was 8" now 10") in my bass amp, but I made sure it was very efficient - net result
- not only is the bass much deeper sounding,
- but because the new driver was more efficiant I now play at a lower volume.

So I am now considering upgrading my other amp (i.e. used for my 6 string) and got to thinking about building a new cabinet that houses two speakers.

I know that connecting the speakers in ...
- series will double the impedance, i.e. 2 x 4 ohms would have an onverall impedance of 8 ohms
- parallel will halve the impedance, i.e. 2 x 16 ohms would have an onverall impedance of 8 ohms

But what I have not been able to get my head around is...
- what will each connection method (i.e. series or parallel) have on the "combined" sensitivity rating?
- e.g. if both speakers are rated at 96db sensitivity, will the overall sensitivity change due to the connection method or remain at 96db?

Since I can get 4 ohm or 16 ohm drivers - which connection method would be best? series or parallel?

in case it is a factor
- the amp is 15 watts into 8 ohm
- I am looking at employing two identical drivers each rated at 96db sensitivity
- 96 db (or higher) is the target for the combined sensitivity

Any help is appreciated - Many Thanks Steve
williewonka
@cj165
I'm sorry, I would call you by name, but I haven't seen it posted.  I came very close to sending this in a private message, but much has been addressed in this thread as well as a couple of others. 
1st... You are extremely knowledgeable and could be if you chose to be, a very strong contributing member to this forum. I personally would love to see that happen. 
You have 7 pages of responses, I did not count the threads, but I would guess maybe 8 or 10.  In those threads I read maybe 20 or so of the post.  In each thread at some point, you belittled someone. 
Odd that one of your threads, you discussed heavily forum etiquette, I believe the thread was discussing if someone needed to prove that they were qualified to chime in on these forums.  
I would say that 98 percent of the members here come here to learn and share the passion of their hobby.  All of us at some point get challenged with our knowledge and being the he men that we are, we seem to have to defend ourselves (except maybe Elizabeth and you attacked her also). I'm not sure if you are trying to prove something or are insecure in yourself that you have to feel superior over others?  Really, I'm not try form a conjecture.  I am really hoping that you are here wanting to contribute and would take it serious that we are all people, all have feelings and all want to be treated with respect. There is no doubt, that I have said stupid things on this forum that can and have been challenged, On the other hand,  there is no doubt, that I have made a difference and contributed to others personal audio issues.  Your knowledge in most area's of audio make my knowledge a pittance,  but I can tell you if the disrespect continues, I will turn you off.  I appreciate your knowledge level and am hoping to appreciate you personally for your contributions.  
I am trying to say all of this in respect.  
Tim (timlub)  
This was posted on another thread today.  I don't think that he is 100% wrong, but the problem of his delivery does not seem to sink in.  Its everyone else. 
@cj165 ... with my pea brain, I had to look up fiefdom syndrome. Thanks for the education there. I picked a few things from your rant for you to consider. 
1. You have not been scorned or ridiculed because  you challenged the resident experts.... You are correct, you are scorned, but it is because of your disrespect.... Overall,  I loved the debate,  Your contribution will cause many people to start studying to get to the last nitty gritty that separated your opinions in the debate. 
2. Others "Command Respect because of their post count"  Their is no Command... I assume that you are referring to Al & Ralph.  They both regularly contribute in a way that has helped countless people solve technical issues in their own systems. 
3. "Followers implore moderators to ban the newbie"   That could get worse, its up to you. I don't believe that anyone would want you to change your opinion and sharing facts.  Its all in the presentation. 
4. "Causing issues to constantly being revisited"  Yes,  issues are often revisited, but it is because, they come up every so often for different people and are often addressed as they come. It has nothing to do with Impeding Technical progress. 
Everyone,  Here is his post from this morning, See Below: 

" Some Audiogoners (no names) start threads after threads which ask the same questions, over and over and over again. They usually get thoughtful responses, but they, the OPs, don’t seem to learn anything or have the motivation to act on any of the thoughtful responses. At best, "they" seem confused or maybe have too much time on their idle hands. " - ps
In Audiogon forums, the ratio of the clueless soothing their fragile egos with endless hand waiving nonsense to the knowledgeable citing proven science and mathematical evidence tends to be pretty high. This scenario is clearly not restricted to Audiogon alone. A high percentage of forums possess posters who "command respect from followers" simply because they have several thousand posts under their belt. The fact that most of their contributions have been hand waiving BS intentionally or unintentionally misleading others doesn't seem to matter. The "reputation" and "belief system" are all that counts. And when a newcomer or "unknown" hits the scene with "facts from the big bad world outside", he/she is often scorned, ridiculed, challenged for credentials, or in egregious cases - the "followers" implore moderators to ban the newbie if the newbie presents facts that disagree with the "grand proclamations of resident forum experts". The end result is often persistent ignorance, in many cases, this ignorance persists with basic fundamental subjects that "in the big bad world outside", have been settled long ago and are considered trivial. This fiefdom syndrome is made worse when utterly unqualified salesmen are allowed to pitch their products without any deference to science or facts. The mere fact that they've been pitching the same old garbage for 30 years is supposed to be enough for the "followers" and sadly in many cases, it is. So just as it is with other forums, at Audiogon, there are a significant number of influences that impede technical progress - causing issues to be constantly revisited that should have been settled long ago.
This is exact quote from cj1965 post:

In theory, half the voltage applied to a driver will result in half the sound pressure in the output (-3db).