The use of any SET requires careful speaker selection. This is because even a higher powered SET will have a fairly high output impedance. For more information take a look here:
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html
But its a really good idea to audition an SET prior to purchase- the more power they make, the less bandwidth they have (such that they may not even be considered hifi). 7-8 watts is typically the limit where they can actually make 20Hz-20KHz. This is why lower powered SETs are always considered the better sounding amps.
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html
But its a really good idea to audition an SET prior to purchase- the more power they make, the less bandwidth they have (such that they may not even be considered hifi). 7-8 watts is typically the limit where they can actually make 20Hz-20KHz. This is why lower powered SETs are always considered the better sounding amps.