I’ve owed both the F502’s and now F502SP’s, I’ve also listened to F703’s and Vintage 15’s. Despite the huge range in price there all recognisable due to the point source technology. All voiced by Dr Paul Mills so their tonally similar also. I fitted the F502SP’s in identically the same place as the F502’s and never had to make any changes – amp tone controls all remain neutral, don’t use any digital correction.
Your lucky someone is letting you borrow the Fyne speakers; I purchased blind. I’ve never listened to any book shelf version of Fynes I can’t comment on those. Your size room the F501SP’s will be on limit for size. My room is 6.5m by 3.7m and both my F502/F502SP’s there perfectly ok, however dare not go any more. I considered the F702 as at the moment some dealers selling x-demo units. The F702 it’s a bigger cabinet then the F502SP and did not want to risk it. Over more weight in the sound, different amplification can help with the weight.
Over sounding harsh / bright. First thing they take some time to run in, Fyne Audio say 100 hours but I felt it was more. As they run in sound becomes warmer and the top end settles down.
The Fyne's are very relieving speakers also the vocal resolution is fantastic that’s down to the point source technology. The issue is because there relieving speaker they show up differences in your front end easily. For example, it’s easy hear differences in DAC’s where some other speakers might mask differences. I think this is where some people think their ‘bright’.
You can think of the Fyne's a bit like a studio monitor. A studio monitor shows up everything in the recording, the issue is if your own gear has issues those will begin to be relieved also. What often happens in audio is an issue in one area can be mistaken for a problem else where.
I post on the Fyne Audio Facebook group, someone recently recorded his Fyne F502 saying they sounded horrible, they did sound horrible it was not the F502, it was issues in his front-end setup. If your finding them bright it’s almost certainly something on your front end that’s causing it.
Both the F502 and SP version both benefit from bi-wiring, Fyne Audio also mention it on their web-site, there is more clarity in the sound. If you have the spare cable, try bi-wiring them.
Almost forgot, the 500SP and 700 series upwards have a 5th speaker terminal, it's to ground the speaker back to the amp to lower noise floor. I've not tried this yet, but on the Fyne Audio Facebook group people are reporting slight improvement in sound when doing this. .
What amp are you using? Also have no experience on the Kef's or Epicon 2's.