Like so many things Hi Fi this amp isn't perfect. There is a lack of deep bass, driving the LS50 with the solid state Creek produced more extended upper bass. One of the things I've never seen mentioned in reviews is bass speed and it does appear to be a little lazy, lagging behind the midrange and treble just a hair. This could be down to the amp speaker combination though, driving a more sensitive floor stander might give better results than the KEF's at the lower end.
I drove the LS-50 for quite some time with my LM219IA and I have auditioned the LS-50 on a number of 100watt + SS beasts.
My main recommendation these days to most people is to do your best to try and buy "synergistic systems" which as an Audio Note owner (system approach) always made logical sense. But the brand doesn't matter - what matters is to buy an amp the speaker maker designs for. KEF didn't design this loudspeaker for SET amplifiers (or tubes for that matter). This is obvious when looking at the measurements or even the spec sheet.
So as an end user, you, are probably better matching with an appropriate amp. SET amplifiers are wonderful - my favorite, BUT their biggest Achilles heels are 1) lack of power and 2) lack of power into varying/low impedance loads. That's why it generally takes rather massive ones to be able to competently drive speakers like the very VERY difficult to drive KEF LS-50 properly and even you are finding it still has trouble.
The KEF is low sensitive AND it has a low and varying impedance load. King Ip of KingKo audio brought the KEF to his store to be a demonstrator speaker and he dumped it almost right off. His amps can't drive them properly even in a small (and I mean very small bedroom sized room).
The 219IA with massive transformers drive them well but I still would not drive them in a larger medium room or larger. And it still wasn't an ideal match by any stretch. Part of this is the lack of damping factor in SET amplifiers. More damping factor = more negative feedback = worse sound. But high damping factor has more driver control into low impedences which is where bass is. Tube changes won't fix this either.
Now of course I am probably far pickier than the average bear (and about different things) and because I generally dislike SS it is a difficult task to find an amp for the KEF. But for the KEF - a larger push pull amplifier in the 70 watt+ range and on the 4 ohm tap is what would be needed. You should have the KEF running from the 4 ohm tap on the LM as well if it is not already.
I guess what I am advising is not to try and shoehorn an amp into a speaker not designed for the amps. For example for $1,995 I would prefer the Spatial Audio M4 which is also 4ohm min 3 ohm but is supposed to be fairly stable. And it is a full ten times more sensitive making it significantly easier to drive.
http://www.spatialaudio.us/hologram-m3-m4
For $2800 the M3 Turbo S has bass to 32hz and is 94dB sensitive. Personally I would take these over my KEF in a second if I were going to use a 20 watt SET amp(or any other amp for that matter). Plus they come with 60 day returns and 20 year warranties. And unlike KEF - made in America which some people seem to like.
http://www.spatialaudio.us/hologram-m3-m4
But they're pretty big (OTOH no stands required). Anyway these are the sorts of speakers that are very good sounding places to start with tubes and aren't horns with their often shouty treble bands.