Stevecham 8-4-2016 10:34 pm edtMany and I believe the majority of tube amps of all kinds will not meet that criterion. For an 8 ohm speaker it would imply a damping factor of 10 or more. Most high quality tube amps seem to have damping factors somewhere between 8 and 2 (with some being even lower than that), corresponding to effective output impedances of between 1 and 4 ohms or so, respectively. (Audio Research and McIntosh tube amps are among the minority having DFs greater than 10, just a bit more in the case of most ARC designs and as high as the low to mid 20s in some McIntosh designs).
... the ~4 Ohm output impedance still seems to a limiting factor; it's difficult to apply the ten-to-one minimal nominal speaker to amplifier impedance ratio often cited in order to maintain flat frequency response.
As you recognize, the 10:1 ratio you referred to will assure minimal variation of the amp's output voltage as a function of variations in speaker impedance over the frequency range. However meeting that criterion is neither necessary nor desirable in many cases. Assuming that the particular speaker does not require a higher damping factor for woofer control, meeting that criterion will be unnecessary and/or undesirable in the following cases:
1)The impedance of the speaker varies relatively little over the frequency range. In that situation the amp's output voltage will vary relatively little as a function of frequency even if the ratio is much lower.
2)The impedance of the speaker is relatively high at all frequencies.
3)The design of the speaker is such that the variations of the POWER it receives as a function of speaker impedance variations over the frequency range should be minimized for best sonics, rather than variations of the VOLTAGE it receives. See Ralph's paper on "Paradigms in Amplifier Design." While such speakers are in the minority, there are many such choices that are highly regarded.
4)Cases where the load impedance seen by the amp can be raised to a suitable level via a Zero autoformer.
Regards,
-- Al