Speaker Efficiency

You may have a low quality alps volume pot ,just look it is a round silver can it is only 2 wipers with a contact spray , once it gets past a certain point it can start loosing resolution , where a resistor ladder type ,or rely much better ,also 

for that speaker the Sonus Faber go down to under 3 ohms you need much more power then that ,the Klipsch are a good fit ,I rebuilt a bunch Klipsch speakers 

the stock internal Xovers are very low quality ,even crites  Xovers which were in it 

not that great the Xover is the ♥️ of the speaker ,having owned a Audio store 

and modding my own Xovers and others for years ,,if you plan on keeping the Forte 

then I highly recommend doing this even theForte 4 cheap Xover parts , once upgraded sounds much more detailed and refined in another league.

the Sonus Faber too could use better Xover parts just unscrew the Bass driver , and look inside , Thst speaker needs a EL34  pentode amplifier .

please remember if pushing a speaker peaks can hit 10 x  more on peaks ,that’s why it’s so e important to match the amp to the speaker .

One thing I forgot Tom mention ,on efficiency of the speaker, Klipsch forte say 99 db actually closer to94-95 db efficiency 

the Sonus Faber you say are 88 DB efficient , for every 3 db it takes 50% more to get to the same volume level  ,the Sonus Faber take over 100%  more amplifier energy to reach the same volume level. Buy a SPL meter on Amazon for maybe$30 for a decent one to know exactly the db you are using ,

then at what db when the amplifier most likely is over driving and Transformer 

saturation is causing it to distort. Either upgrade the Klipsch ,or buy a much bigger amplifier,  the Quality and size ofthe transformers means Everything 

if you want excellent quality without major league pricing, Lookup 

Radu Tarta , one ofthe best custom Amp,preamp designers in all NorthAmerica , he uses Quality like Lundahl chokes and Transformers  , in the  decware a smaller type lower quality , Radu has built several Amplifiers and  preamps for myself .

not cheap but built to last a lifetime he is the-best especially SET amps

which are 8 watts ,or a class A parallel feed using4  power tubes you can get 18 watts . All depending on your plan and budget. Much more$$ though 

for sure though the SF speakers Waay underpowered.