Here at MayosMusic, in addition to offering great guitar, bass, ukulele, banjo and autoharp lessons, plus complete luthier services that include guitar, string instrument and amplifier repair/restorations, we are also a great resource service. Consider: the two most important links in your sound chain are the first and last, Pick-Ups and Speakers. So let's talk about speakers...
The speakers in your amp are the transducers that translate the electrical signals of your equipment into the sounds we all hear as the music you play. Players like to change tubes, strings, instruments, picks, even internal components like capacitors and such in an attempt to find the magic mix that is "their" perfect tone. Well, speakers can often be the easiest and quickest (though not usually the cheapest) way to change tone subtly, or even drastically.
Ratings - Speakers are rated in several parameters and tonal characteristic descriptions by manufacturers and/or users. Let's start with the measured parameters:
Power - Speakers are rated for power handling in units called Watts, with most showing the maximum or peak power they handle. Some will show their RMS (Root Mean Square) which is the equivalent power if the signal was DC. Some manufacturers will state their "music" power rating, which is their rating for how much power of random signal (since music is not exactly a perfect sinusoidal waveform) they can handle.
Impedance - Impedance is the vector magnitude of the combined DC resistance (Ohms - Ω), and the frequency dependant inductance (mH). It is usually posted as "Nominal Impedance" which is the average impedance over the signal range. Some manufacturers will display their speaker's minimal impedance, which is helpful for safe and proper connection to power amplifiers.
Frequency Response - The measured minimum and maximum frequencies the speaker can reproduce is the frequency range. Hopefully it is displayed at a given +/- dB range, but is not always (I'll go over that rating in a second). Frequency range is measured in units called Hertz (Hz), which represents what used to be called CPS, or Cycles Per Second.
Efficiency - The final common speaker parameter (at least for the music industry, those in other industries might concern themselves with more measurements) is its efficiency, also referred to as Sound Pressure Level (SPL). The efficiency of a speaker means how loud it gets. They typically measure this in a given cabinet connected to a 1 Watt signal and measured at 1 meter with a control microphone. Then they sweep the signal or just rate it at 1kHz, or 800Hz, or some other standard. The measured sound pressure is the efficiency, and rated in deci-Bells (1/10th of a Bell) noted as dB. Now every +3dB is twice the power, and -3dB is half the power (very important). And the average human discerns twice the volume to their ears as being around +7dB to +9dB.
Types - Most speakers in instrument amps are going to be highly efficient paper cone "dynamic element" or "dynamic driver" speakers, referring to the familiar round cone that moves in and out. Now in Hi-fi, there are many more types/designs of speakers, such as electrostatic, electrets, compression horn, ring radiator, planar, ribbon, piezo-electric, and a few others. Compression horn drivers are common for highs and sometimes mids in PA speakers, and in some bass amps. And piezo horns are common in cheaper bass amps for the highs too. Some Acoustic guitar amps are now using ribbon or even electrostatic (though usually mislabeled and are actually electret) drivers for the highs. Acoustic amps with ribbons and such will sound very nice and often described as airy, effortless, and with great subtleties.
Tone - Just as pedals and amps can add distortion into your tone, via transistors, op-amps, chips, FETs, and tubes (both pre-amp and power amp distortion), speakers can impart their own distortion to the mix. Sometimes that distortion is subtle and referred to as "Timbre" or tone, which is the amount of altered sound across the frequency spectrum from electrical signal input to the sound output. However, most people simply refer to this as the tone, character or timbre of the speaker. The more noticeable and obvious distortion types are the clipping/overdriving/compression/breakup distortion that is usually collectively referred to as the "distortion" character of the speaker. This type of distortion is made when a speaker is driven close to or at its maximum power handling.
AlNiCo - Alnico is one of the earliest magnets used for speakers. It is light weight and fairly strong. Ceramic magnets came along a little later, and while not quite as strong per weight, they are far cheaper to manufacture. All other speaker factors being equal such as magnet size or strength, voice coil diameter and gap, number of windings, wire type and gauge, cone material and weight, etc. the two will inherently sound different. This is because AlNiCo magnets are an alloy magnet, verses ceramic magnets, and they are easier to demagnetize. When a voice coil moves from a signal, the current through the wire coil is generating a magnetic field in opposition to the permanent magnet (thus movement and sound production), and when this happens, the resistance from the spider, the surround, the momentum of the weight of the speaker, and the air resistance against the cone all act to prevent movement, which means the voice coils magnetism is trying to cancel or squish the opposing force of the permanent magnet. When this happens, AlNiCo speakers react to it, and their permanent magnet is magnetically bent and compressed and this causes the speaker to become less efficient, at its extremes, which means that it exudes an amount of natural compression to the signal being translated into sound. This does not exist in the same way with ceramic magnet speakers.
Ceramic - Ceramic magnets don't compress, so the voice coil instead moves all the way to the ends of its mechanical limits, which can sound harsh or edgy when pushed hard. Thus the general rule is that AlNiCo speakers sound warmer, and smooth, which ceramic sounds bright and harsh, when listened to at loud levels. You can to a certain extent liken the ceramic speaker to a solid state amp, and an AlNiCo to a tube amp, in that their perceived power/headroom and natural compression/distortion are comparable. However, when it comes to choosing which type to use, it all comes down to finding the right balance of all components, to achieve the end result you desired.
Neodymium - For the most part, Neodymium speakers sound and act like AlNiCo speakers, usually with higher power handling and higher prices, more or less. While they handle more power, they unfortunately have a less forgiving damage limit when it comes to over heating. Neodymium is a highly sensitive magnet, which is what makes them sound closer to AlNiCo, however, once they overheat, they are permanently damaged. This is why most "Neo" speakers have a lot of heat sink fins on the magnet structure. So while "Neo" speakers are somewhat delicate, if played within their parameters, they can offer a tremendous savings of weight, especially in multi-driver bass cabs like 4 x 10" or 8 x 10".
Bass amps will want a sealed or ported cabinet, and not an open backed cabinet, as they are looking for bass control and focus, and with more bass control, you can increase the power rating as compared to a looser speaker movement in an open backed cabinet. You might find your speakers and amp are both great but that the cabinet design is not. Maybe opening up, partly opening up or sealing off a speaker cab is the change in tone you are looking for. Some aftermarket speaker cab companies offer convertible cabs that can be configured as open or closed back.