jbuhl
The basic question is: from a specification standpoint which speaker pairs up with the Stellar 300 best?
Sorry, but to me neither speaker will be at their best with the Stellar, it will work into them and probably sound fine, but probably not extract the best from either. >2ohm loads are not Class-D’s forte.
I don’t think big wattage is the answer here as both speakers are over 90db, a 50-100w amp will do, the issue is the amp needs to supply good current into the low bass load as seen by it (below) to keep it from becoming a tone control.
The Quad S5 has a demanding load in the low bass.
As measured by Keith Howard of HiFi News:
" EPDR (equivalent peak dissipation resistance) to fall to a minimum of 1.4ohm at 111Hz, making the S_5 a more than a typically testing load for this size and class of floor stander "
The Focal Aria 962 is also a hard load in the bass as can be seen by this graph from 80hz to 300hz. 2.8ohms and also a -55 degree phase shift, which equates to a EPDR well below 2ohms
https://www.stereophile.com/images/1114FA936fig1.jpg
A Class-D, Tube or Mosfet is not my first choice of amp for loads like these to get the most out of the low bass of these two speakers.
I would prefer to see a linear amp with lots of bi-polar (BJT) output devices and big power supplies, to push current into that bass to keep it under control and flat without becoming a tone control..
Cheers George