MoFi Sourcepoint 8 or Q Acoustice Concept 50s

@lanx0003 well luckily their amp isn’t a high output impedance tube amp. it is solid state AB amp with high damping factor so impedance mismatch isn’t going to be an issue here

@maam522a get the Sourcepoint 8. 

Have you had a chance to listen to either pair of speakers at your local dealers, or at a show?  Me, personally, I wouldn’t purchase the most critical component in my audio system (speakers) without first being able to hear them in person somewhere, somehow.  Seeing as it my be impossible for you to be granted an in-home audition, I would even travel a reasonable distance in order to audition.  Once I decided on the speakers, then I’d purchase, trade, exchange, amplification that synergizes with your speakers of choice.  Or, you could buy based solely on what you think would synergize best with what you already have.  You have subs, so you don’t have to worry about low bass extension.  Good luck.  Happy listening.         

@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.