First Watt with KEF LS50s

If the powered subs are doing the work below 80 hz my guess is it would be suitable. 

As great as the First Watt amps are, I think that they might be a bit underpowered for the LS50s to sound their best. So I'd go for the more high powered models in that line.

I'd also recommend talking to Mark at Reno Hi-fi if you don't have a dealer in your area. He has a good return policy that will permit you to hear for yourself. 

First watt is best for high efficiency speakers which kef Ls 50 is not. Crossing over high at 80hz will help a lot.  Could be suitable but probably still not optimal.   Use of subs will make for better results in larger room for sure. 

The F6 should easily drive it and sound wonderful

CRossing over at 80hz still leaves a fair amount of bass up to the low power amp and inefficient speakers. I’d predict sound with less impact or thinner than might be possible otherwise. If you check the audio frequency chart you’ll see upper bass comes into play quite significantly with most musical instruments. You could still well get into clipping sooner rather than later meaning distortion and/or rolled off dynamics at higher volumes. A smaller room would be best suited.

Bottom line is it may be "suitable" but less likely to be optimal.

Whereas two powered subs, ls50s and a high power and current amp would probably perform optimally in a larger room. Class D amps would be a great option to consider.

With no subs. ls 50s are better suited to smaller to modest sized rooms.