Nope, this is where your flaw in reasoning is.
Let's assume this amp is capable of outputting about 4.5V of clean drive voltage into a 4Ω load, which is 5Watts.
If you attach 4 speakers to this in series parallel, the two parallel paths have by nature the same voltage applied to them. However each parallel path consists of 2 series speakers, which act as a voltage divider. Therefore each speaker will only have 2.25V applied across its terminals, before the amplifier starts to distort.
So by using 4 amps, you avoid having to put speakers in series, and therefore each speaker sees the full voltage put out by the amps. Double the voltage = +6dB.
And as for the increased efficiency? Hornresp simulates this fairly well, you can get a theoretical 6dB gain for certain transducers at certain frequency ranges, but it's usually much less. This is quite clear if you consider what the long term consequences would be of each quadrupling of speakers being a 6dB gain in sensitivity. If you start out at a baseline 91dB/2.0V speaker, using four speakers would put you at 97dB, sixteen speakers at 103dB, 64 speakers at 109dB, 256 speakers at 115dB. And 115dB is 2 acoustic Watts. Conservation of energy dictates that you're not going to get this out of 1 electric Watt. Any improvements in sensitivity are due to acoustical impedance coupling improvements, which is only possible up to a certain degree.
However one upside of using four speakers series parallel instead of one single unit with the same sensitivity throughout most of the band, is that you're getting this sensitivity at half the excursion of the drivers - lowering the distortion of each speaker.
Yes, I'm aware of that. It doesn't alter the fact that any minor advantages that may be obtained will need to be set against the not inconsiderable size and cost of requiring 8 SET amplifiers, and that's just for the wideband setup. This could be tested cheaply using some class D modules or similar to confirm one way or the other, but personally, I'd lay good odds on it not being worth it in practice. A couple of percentage points against building, housing & running 8 SET amps? Hmm. TBH, I like the idea in the abstact for its excess, but with my practical head on, I'd be more inclined to put the money into something that should make a more significant change. YMMV of course.
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Something to think about:
By using multiple amps, you will be using multiple smaller transformer cores instead of one bigger one.
May be you could also consider instead of having multiple amps individually powering each driver, the other way round would be to bridge all the amps and drive all the drivers in series.
I think the reason that this isn't commonly done is that it requires a lot of materials and not cost effective. Multiple cables, binding posts, OPT, output tubes, input tubes, RCA jacks are needed. Instead of paralleling all of those, assuming that you intend to use 300B, personally I would use a single pre amp box that have the guts to drive all 4 output stages.
By using multiple amps, you will be using multiple smaller transformer cores instead of one bigger one.
May be you could also consider instead of having multiple amps individually powering each driver, the other way round would be to bridge all the amps and drive all the drivers in series.
I think the reason that this isn't commonly done is that it requires a lot of materials and not cost effective. Multiple cables, binding posts, OPT, output tubes, input tubes, RCA jacks are needed. Instead of paralleling all of those, assuming that you intend to use 300B, personally I would use a single pre amp box that have the guts to drive all 4 output stages.
One advantage of separate amplifiers for each driver would be better electrical damping, especially vs running drivers in series.
That advantage may be cancelled out by using a transformer-coupled tube amp, in which case Gainclones would be worth trying as a cheap, relatively easy experiment. You would then be able to try both approaches (multiple amps or series/parallel) and see if the difference was audible. The multi-amp approach is one I fancy trying with the Jordan 50mm speakers some day, with the added bonus that it would make it easier to filter the signal to individual drivers if required.
That advantage may be cancelled out by using a transformer-coupled tube amp, in which case Gainclones would be worth trying as a cheap, relatively easy experiment. You would then be able to try both approaches (multiple amps or series/parallel) and see if the difference was audible. The multi-amp approach is one I fancy trying with the Jordan 50mm speakers some day, with the added bonus that it would make it easier to filter the signal to individual drivers if required.
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