I have both: a pair of Class A monoblocks and a pair of Class A 2A3 based SET monoblocks. My impression of which is "best" depends on what I'm listening to and what kind of presentation I'm looking for in enjoying music - so all theories aside...
A good example is in how the piano is presented. With the tubes, the so-called distortion gives the piano a warm, pleasing sound that makes it easy on my ears. Very fluid, warm and relaxing. The SS monos (ML 20.6's) takes the piano apart. It gives you everything from the attack on the keys and string hammers, the pedal, the sounding board - everything. It presents a more realistic "you-are-there" sound where you can tell the difference between a Yamaha and a Steinway and the venue it's playing in. It also gives you a the presence of the pianist himself. So if you like to recreate a live performance, the SS mono's do a better job but take away some of the sweetness that the tubes give. Analytical? Yes. Unlistenable, definetly not. Just different.
Other types of music such as orchestra, rock, jazz, vocals, are presented the same way. The orchestra, for example, is better presented with the SS mono's because imaging, depth, instrument timbre is recreated in a way which makes the performance more realistic. The ss has power in spades to lift all the frequencies of the orchestra (macro and microdynamics) so that they are at the same relative loudness levels as the performance. The tube amps will strain under the crescendos and may not catch all the frequencies equally, but it will present an overall sound that is more pleasing, especially during low, melodic passages.
All in all, it all depends how and what you like. Sure, I'd say get both but that's going overboard. You can't go wrong with a pair of good SET's if you are willing to endure their finicky nature, if you don't need wall-shaking power, and are more interested in a pleasing sound rather than acoustic gymnastics.
