LS-50 is a much better speaker than any LS-3/5a - I reviewed arguably the best remake of them in the Audio Space LS-3/5a. The LS50 has far better bass response and doesn't suffer step response from woofer to tweeter. The LS-3/5a is good but it's really not in league with current standmounts and not just the LS50. With the Subwoofers designed for the LS-3/5a that somewhat changes but then the prices rise to $3000.
That being said your Fisher is a classic tube amp of not a whole ton of power - that in itself is okay but how beefy and boig are the power supplies. Does the Fisher have a 4 ohm tap? If it does it will be fine.
Both speakers are designed for a small room and presumably head banging isn't your thing. KEF will be fine.
I will give you my alternate speaker - The lesser known but criminally unfortunate lesser known, Audio Note AX Two. The Audio Note AX Two was designed by Andy Whittle of Rogers LS-3/5a fame (and also formerly of Celestion). The AX Two's list price is $1200 - it doesn't have the bass of the LS50 but it's still MUCH better than the LS-3/5a and the AX Two sounds more cohesive than the LS-3 and has a bigger presentation than the KEF. It's not an easy choice - the AX Two has a nicer treble than the metal tweeter in the KEF - so there are pros and cons.
The AX Two is better at low volumes the KEF is better at louder levels with bass impact. The AX Two is 90dB sensitive 6 ohm and likely doesn't drop below 5ohms. MY AN J/Spe is 6ohms never drops under 5 making them every tube amp friendly and since Audio Note's partnering amp for the AX Two is the I-Zero integrated at a whopping 8 watts - you can be sure your Fisher will drive them easily.
Another plus since you mentioned it is that the Audio Note AX Two is hand made in Denmark with real wages using Danish VIFA tweeter and woofer. The internal parts are made by Audio Note in England.
The downside is that the wood body is nothing spectacular but they still look more retro than the KEF - although the KEF is very nice in person - piano black finish - space age driver - the KEF feels a lot more substantial. Still the AX Two is cut from the same design cloth and is like I say criminally under the radar - perhaps because for the last 13 years they've never bothered advertising the speaker.
The AX Two is obviously the smaller one from my picture - the big ones are the Audio Note E (based on the Snell E).
I like em both equally - they bring different strengths to the table. I think both are a full letter grade higher than the LS-3/5a versions.