As for me, serial connection is a waste of money. Actually, the second driver (the second and each subsequent one) in the line does not bring anything useful. It seems to me that if someone gives two transducers, he does it for a specific purpose and having an understanding of audio as such, it is not difficult to find this goal - unfortunately the serial connection has nothing to do with it.
. It must be remembered that the use of 2 transducers in one path playing the same band on one filter and in one housing, and often in a common chamber, carries the risk of several undesirable phenomena, such as wave interference, etc. I do not want to and two are not strong enough, but it seems to me that for the purposes of this topic it is enough to say that when using at least 2 transducers (whether in series or in parallel), we risk the occurrence of undesirable phenomena. If so, the profit from having them should be some considerable so that risking yourself would make any sense.
Well, now we come to what we get from the use of other transducers?
Certainly, two of the same will not sound better than one - you can combine with different applications and thus improve the sound, but the combination itself will not be better without any special measures.
If so, what do we gain? Well, in a parallel connection, we get an additional 3 decibels due to the doubled surface area of the membranes. In the parallel one, the impedance of the loudspeaker will certainly be reduced by half and therefore the load on the amplifier will change, which will give us another 3 decibels - in total there is already 6 decibels of efficiency increase. In addition, there is an increase in power by another converter, i.e. the ability to pump 2 times more power and obtain further decibels. You have already accumulated 9 decibels and this is something worth fighting for when someone likes it loud
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And how does the serial connection relate to this? Well, what if we add a second transducer and get a gain of 3 decibels on the surface of the membranes as the impedance of the set will double and it will choke the amplifier twice and the gained 3 decibels will be leveled. Of course, the power of the loudspeaker will double and it will be possible to pump in 2x more power and thus gain, but is it sure to gain? In the end, the amplifier will be needed quite a bit, and as you know, this costs money. Maybe instead of two, buy one twice as expensive with better sonic qualities - I do not mention greater efficiency because it may or may not be, and for me the efficiency of the home set is of secondary importance and only the sonic qualities count.
Some people see the advantages of increasing the impedance twice and "fitting" to an amplifier that, for example, does not support the lower impedance that one would have. You can and yes, but ... is it worth it, everyone has to answer for himself. I would change the amplifier there
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There is one more option, as I mentioned above, i.e. various applications of the converters in the cabinet, i.e. separate chambers and, for example, various tunings - in a word, treatments that emphasize some parts of the sound spectrum and aim to improve the sound of the loudspeaker. If someone succeeds, this is an argument in favor of using a larger number of transducers - both in series and in parallel, but the advantages of the parallel probably outweigh
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Besides, I prefer to buy one 2 timesmore expensive (of course I mean better, but the rule that more expensive is better, unfortunately, does not always work) than two half cheaper. If I had to decide, it would only be connected in parallel.
