MoFi Sourcepoint 8 or Q Acoustice Concept 50s

@dynamiclinearity @kofibaffour   Well, it’s not as immediately obvious to me as it is to you two (and probably many others) why a higher-damping-factor amplifier is more resilient to frequency variation. I had to spend some time thinking it through and convincing myself.

By the voltage divider law (for two series resistors), Vi = Vo Zi / (Zi+Zo), where i = input (source) and o = output (load).  For a tube amp, assume Zo = 2 ohm. When impedance of load (on speaker) rises from 8 ohm (150hz) to 75 ohm (1.9khz), the voltage drop Vi/Vo = 8/(8+2) = 0.8 and the change in volume level = 20log(0.8) = -1.94dB, and then transitioned to 20log(75/(75+2)) = -0.23dB.  This transition leads to an increase in sound pressure level of 1.71dB.  The perceived increase in loudness = 2^(1.71/10) = 13%.

By contrast, for a SS amp, assume Zo = 0.05 ohm, performing the same calc. leads to an increase in SPL of 0.04dB, the perceived increase in loudness = 0.3%.  It’s immediately clear that the perceived sound-level variation caused by this impedance transition is much smaller for a solid-state amplifier than for a tube amplifier.

On the low end, the impedance transition from 8 Ω to 21 Ω results in about an 8.3% increase in loudness for a tube amplifier. It’s still an increase, but the perceived change in bass loudness is relatively less noticeable to the human ear. That might be why no attenuator is provided for it.

Obviously it's OK to use low damping factor amps even though there is frequency variation. Many people love tube amps and many love Pass amps(solid state with low damping factor due to no overall feedback). A good place to see this is John Atkinson's graphs of amp frequency response into a sample reactive speaker load(and every speaker is reactive, not resistive like the load used to test amplifier frequency response). As a sort of rule of thumb a damping factor of 20 yields variations that can be measured but are very small. But tube amps often have damping factors of as low as 2 to 8(fairly high for a tube amp).

Of course amps have other characteristics than frequency response which is why some like low damping factor amps. Also note that damping factor varies with frequency and will be lowest at say 1 kHz. And it will be higher at frequency extremes, sometimes significantly. You need to know what frequency it's measured at. It may be OK at 1 kHz but high enough in the bass to react with bass impedance peaks.

With your current amp and, I assume, you are keeping the Rhythmik 12"Sub, the SP-8 would be my choice.  I like the Mofi speakers I've heard.

If tube amp DF goes down to 2, meaning its output impedance is as high as 4 ohm.  The perceived increase in loudness is going to be 26% in high frequency.  You need attenuator to tame it down or avoid using such tube amp on speaker with char. imp. of this behavior.  

@lanx0003 @dynamiclinearity - Thank you for your insights and guidance.  The Aragon Amp has a Damping Factor of greater than 500 according to the original Stereophile review.  It is dual mono (designed by Dan D'Agostinto) who designed the Krell amps and provides very high current and will play near 800w into 2 ohms.  Another consideration - for the 8s - is that i can actually place the acoustic center at or a little above ear height which puts the top of the speaker at 45 to 47 inches.  I am 6'-2".  I can point them into the room rather than directly at my listening position - attenuating any treble brightness .  As for casual listening, they are high up enough to play over some very low slung mid century modern chairs that I move when it is just me listening.