Liquid Amps

@atmasphere explains well why it's a little tough to find "liquid" SS amps.

However, there are many issues with tubes too.  I have both SS (McCormack, McIntosh, digital amps, and others) and tubes (ARC Reference), so it's not like I'm prejudiced toward one or the other.

While people are offering nice suggestions and commentary, they are jumping the gun.  We must first know what speakers you are using.

Many speakers have a definitive preference toward tubes or SS.  Thus, any recommendation is primitive without knowing what speakers you are using with this amp.  20 SS watts won't cut it on certain speakers...conversely, certain high power or high gain SS or digital amps will have problems sounding "liquid" with certain speakers (i.e. horns). 

It's actually something that can often be measured.  For example, the output impedance of a certain tube amplifier can pull down levels in higher octaves on certain speakers while boosting the lower octaves.  This could be a good thing and certainly make the sound seem more "liquid".

In short, you really have to let us know what speakers you are talking about.  In my systems, modified McCormack amp are the most "liquid" sounding with my particular speakers...even moreso than my ARC reference amps.  However, I wouldn't guarantee it would do the same for every set of speakers.  Some may like the strengths/weaknesses of tubes more than the strengths/weaknesses of SS.  (I've had Spendor speaker in the past that loved tubes far more than SS.)

All this being said...I don't think the BIGGEST problem with systems not sounding "liquid" is the fault of higher order distortion in SS amps.  I don't think Atmasphere made the claim that it's the "biggest" problem either, so we may not even disagree per se.  I just want to point out that IME the biggest problem with systems not sounding "liquid" rests with the speakers and source first...not the amp.

It's really amazing what can be accomplished when you get the right pair of speakers and feed it an impeccable source.  An amps strengths/weaknesses can still be apparent, but it become obvious that it's not the driving force behind problems unless you have a really poor amp (or poor combo with a preamp). 

If you're doing digital, the biggest impact on liquid sound is getting rid of jitter.  Focus on the server/renderer even more than the DAC nowadays.  If you're doing vinyl, you probably already know how big an impact the cartridge and phono preamp can make.  Address those things first before thinking a "liquid" SS amp will solve your problem...it can only do so much.

Atmasphere's suggestion for weighting the type of distortion is excellent.  What's even more important than the type of distortion is where is the distortion located.  Our ears hardly care or notice distortion in the bass regions relative to others.  Most subs have horrendous distortion and are driven by cheap digital amps.  Thus, it's not only the type of distortion that matters, but where the distortion is present too.   

That's why I stand by my original comment.  The speaker and amp purchase cannot be done in a vacuum.  The speaker and it's crossover will play a large role in how the amp behaves and what sound results.  Without factoring in this connection, any universal suggestions are primitive.

As I mentioned, I own lots of stock amps and they can perform well with the right speaker.  However, one can't say arbitrarily that a small amp (like a 10 watt single ended amp from First Watt) that sounds liquid on certain speakers, small rooms, types of music, and/or volume levels will "smoke" a balanced 150 watt amp customize modded for your particular speakers and using the latest and greatest parts and technology (like a modified McCormack circa 2018).

It's really not even debatable and evidences a bigger problem within the hobby than distortion measurements.  Most people buy equipment (and ask/comment online) in a vacuum.  They totally forget about synergy and that's one reason why you consistently hear crap from so many systems with "high pedigree" gear.  It's also why you see such a cottage industry for things such as Autoformers that legitimately can help in some situations because certain speakers and amps are so poorly matched on their own.

Also, the other reason (which I mentioned previously) is that most people skimp on their source which thus creates garbage that no amp or speaker can fix. 

Plus, very few people own a truly full range high end speaker and listen in a room that optimally reproduces music. 

All these other things I've mentioned will impact a "liquid" sound far more than an arbitrary amp recommendation and is probably the REAL CAUSE for the original poster's requested suggestions in the first place.