Re: speakers
you can use as high a wattage speaker as you like. Speaker wattage is purely an indicator of how much energy (measured in watts) in the form of heat it can throw off and continue to operate safely.
Bear in mind tho, that the higher wattage speakers are built more robustly than their low wattage counterparts. Due to the use of larger diameter voice coils etc. Higher wattage speakers generally tend towards having less treble detail (kinda like high output pickups vs low output pickups). However there are plenty of low wattage speakers that are muffled and dark, and high wattage speakers that are clear and articulate - because there is a lot more to the story.
When it comes to speakers it is not the wattage that tells you how loud it is. The wattage rating just tells you how much you can put into it without blowing it up.
It is the speaker sensitivity that tells you how loud it is. This number is usually in the 90 to 105db range.
As far as this goes, with one watt of input power, you can expect to get around 100db of volume. Every time you double the wattage input you get an extra 3db of volume increase. So 2 watts will give you 103db, 4 watts 106db, 8 watts 109 db and 16 watts 112db.
As such, with a 15 watt amp using a 100db speaker you can get just as much volume as a 30 watt amp using a 97db speaker. However, if the 100db speaker is only rated to 15 watts (such as the clestion alnico blue) you could potentially blow it up with a 30 watt amp.
The resonance (75hz in your case) is the frequency at which a speaker most readily responds to an electrical impulse. You can generally say that a speaker with a lower resonant peak will have a darker voice than one with a higher resonant frequency. Resonant frequency is just a part of the puzzle that creates the sonic character of a particular speaker. You don't have to worry about choosing a speaker based on that because there are many other factors that have a large bearing on tone. A good way of at least getting a rough idea of what a speaker will sound like is by looking at its frequency graph and compare it to the speaker you have now.
When you choose your new speaker just make sure that:
1. the impedance of the speaker matches the impedance of the amp's output. Most common amps have an 8 ohm out for a single 8 ohm speaker. the 16 ohm output is generally for two 8 ohm speakers run in series. However, your AC15 amp is designed to run with a single 16 ohm speaker - which is why it comes stock with a 16 ohm speaker. So its an easy one...get another 16 ohm speaker to replace it.
2. The wattage rating of the speaker should equal or exceed the wattage rating of the amp. As long as you have a speaker rated for at least 15 watts, you can't go wrong.
3. Try to choose one that is known for sounding good in open backed cabs. Closed back cabs provide a nominal +6db increase in bass frequencies, so in the case of your greenback, they are popular in closed back cabs (think the old marshall quads where 4 of them can handle 100watts and provide plenty of thump), but alone and in an open back cab they can tend towards being a bit light on in the low end.
