The class of amp is completely independent of whether it is valve or SS... Valve amps are much more commonly class A than SS amps (SET's can only be class A) and especially when 50WPC ish or less. SS amps are rarely class A simply because it costs so much... when class A the £/W will be similar to that of a valve amp (as will the size, weight and heat produced!) and people kind of expect SS amps to be much cheaper, Watt for Watt, than a valve amp. This IMHO gives valve amps an "unfair" advantage and is what can sometimes hold back the performance of SS amps.
I could go on all night here and make this the longest post on record in comparing and contrasting the damping factor, frequency response and distortion performance etc etc of valves V SS and all the factors that define the parameters... don't worry I won't!
IMHO the best examples of valve and SS amps will sound pretty much the same in most areas but SS will always have an advantage in the bass due simply to the lack of an output transformer. I don't include SET's here as, whilst many people swear by them, they are intrinsically high distortion and poor damping factor (the latter due mainly to lack of feedback and the former partially) and this will give them the warm, lush sound they are known for and which many people love... but is ultimately a distortion and inaccuracy and so not as hi in fi as other types. The best SET I've personally heard BTW is Gazjams Long Dog Audio unit. See Lurcher (Nick) for one of these...
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Modded Thorens TD150, Audio Technica AT-1005 MkII, Technics EPC-300MC, Arkless Hybrid MC phono stage, Arkless passive pre, Arkless 50WPC Class A SS power amp, (or) Arkless modded Leak Stereo 20, Modded Kef Reference 105/3's
ReVox PR99, Studer B62, Ferrograph Series 7, Tandberg TCD440, Hitachi FT-5500MkI, also FT-5500MkII
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