Ohm help for WEM Control ER 15 vintage valve amp

[Jazz P Bass]

Yup.
We covered the bases.

The answer is in the OT.

To know the ratio is to know the answer.

I would hope the manufacturer knew better & picked an OT right in the middle of 8 & 15 (sic).

[Enzo]

I have read the Old Testament, and I don't recall impedance getting a good discussion there.

[Mickey]

From the schematic, the amp is designed to run into a 15 ohm speaker. If you run your speakers in parallel you'll end up with 8 ohms. Now when that is reflected back into the primary of the OP transformer the primary impedance will in fact be shunted so that it's impedance will be reduced. This in turn will increase the primary current flow which will most likely shorten the life of your output tubes, it may also shorten the life of the OP transformer as well as that of the PT. Audio transformers are not usually just slapped together, their impedances take into consideration the number of turns, the gauge of the wire and in some cases the polarity of the windings.
Now if you wire your speakers in series you would end up with an impedance of 32 ohms. This will reduce the available volume and will also reflect a incorrect load back into the primary of the OP transformer albeit not as damaging as would a 8 ohm parallel speaker connection.
A simple analogy is of using the same speaker wiring but connected to a solid state amplifier. With an 8 ohm load instead of the designed 15 ohm load the output stage of the amp would probably last a few seconds, however if a 32 ohm load was connected the amp would lose some of its volume and probably would also have an altered frequency response but it would operate indefinitely albeit not sounding as designed.
Harking back to my valve/tube repair/service days it was not uncommon to have a call back on a TV in which another service person had fitted the wrong audio output transformer, usually the output valve/tube had failed shortly after the first call out because of increased dissipation due to incorrect reflected impedance.
I think the level of volume would only have minimal affect on longevity of the output stage if an incorrect speaker impedance was connected.

[pdf64]

The OT secondary may have an 8 ohm tap / terminal, I think that was fairly common for Watkins / WEM.
So have a good look to see if there's an unused terminal on the OT.

If not, as the amp is cathode bias, I would err on the side of an 8 ohm load, rather than 32, due to its self regulating nature tending to compensate for excessive plate current.