90db 4 ohm with tubes?

Assuming you are referring to the current Diamond DMD version, the impedance curve shown in the review Samhar referred to indicates an impedance rise to around 28 ohms in the upper mid-range, some smaller impedance peaks in the deep bass region, and, according to the text of the review, 4 ohm impedance minima with "overall measured impedance [of] 6.5 ohms, so this is nominally a 6 ohm loudspeaker."

The rise to around 28 ohms in the upper mid-range will result with your amplifier in somewhat greater emphasis of those frequencies than would occur with nearly all solid state amplifiers, and also with some tube amplifiers (those having lower output impedance than I believe yours does). There will also be a bit more emphasis of frequencies in the vicinity of the two impedance peaks in the bass region.

I don't know whether or not with those particular speakers and your particular amplifier that would be good, bad, or something that comes down to a matter of taste and room-matching. But it's something to be aware of.

Regards,
-- Al

looks like the sound will sacrifice tonal balance.
upper mids emphisis will create fatigue.

As a follow-up to my and Marakanetz' earlier posts, I found the following statements in John Atkinson's report on his measurements of the V12, which appears to be generally similar to the V12R aside from differences in the input section:

... allowing for some variation in the calculated output impedance with the level and load impedance used, we're looking at source impedances of 5.5 ohms (8 ohm tap) and 2.8 ohms (4 ohm tap) in triode mode. While these are both high, they are exceeded by the ultralinear figures: 12 and 6 ohms, respectively! Fortunately, the impedances don't change much across the audioband, but, as figs.1-3 reveal, there will be a large and audible change in frequency response depending on which loudspeaker is used with the Cary and which transformer tap and mode of operation are used....

... I was concerned by the highish levels of HF intermodulation and the very high source impedances, particularly in ultralinear mode. Unless you really need the extra 3dB of dynamic range you get from ultralinear operation compared with triode, and the extra 3dB you also get from the amplifier's 8 ohm tap compared with the 4 ohm tap, my advice is to use the CAD-280SA in triode mode with the speakers hooked up to its 4 ohm outputs.

Caution would seem to be in order here. I would not advise pairing such an amplifier with a speaker having such wide variation of impedance as a function of frequency, without first carefully researching the results others have obtained using those speakers with the same or similar amplifiers, AND carefully auditioning the speaker with that specific amplifier.

Regards,
-- Al

Phooey I say, If the amp has a substrantaial power transformer (it does) and good output transformers (it does) then you can play you music through those soeakers. You will have to decide if you like the sound however-you may or may not- but I say it won't be the amps fault. I have lower wattage tube amps driving speakers that like to dip south of 4 ohms which sound absolutely fabulous. I am certain the math would say no... but my ears say yes,

Dr. M, the concern is NOT the speaker's 4 ohm or whatever nominal impedance. The concern is with the wide VARIATION of that impedance as a function of frequency (from about 4 ohms to about 28 ohms according to the measurements of the particular speaker that were cited earlier, with the 28 ohm peak occurring in a critical part of the spectrum), and the INTERACTION of that VARIATION with the amplifier's high output impedance.

Yes, if the sonic results turn out to be poor "it won't be the amp's fault." It won't be the speaker's fault either. It will be the result of a mismatch.

Regards,
-- Al