The difference is to do with the way they distort, I am not 100% on the physics but its something like (bad) digital distortion ends up with a square wave, and harsh harmonics which interfere with the pleasantness of the sound, whereas tube distortion provides rounded corner-wave and a harmonic distortion.
Whether the Valveking is a great example of tube distortion I have no idea; it could be that the ME-50 actually is a better digital than it is a tube.
Tube distortion gets more complex in that the most harmonic form comes from distortion of the
power amp valves i.e. at very loud volumes, or towards the limits of the amp's output. So the preamp (pre-gain) distortion on a tube amp is not as good a sound as the hi-volume. A 4w tube amp is the solution here.
There is an amazing site somewhere on creating distortion with EQ pedals, distortion pedals and a distortion channel on an amp.
Basically you can use EQ in between these stages:
guitar > EQ >
distortion pedal > EQ > preamp stage > EQ (from amp or external effect) > power amp
all 6 of those stages feed each other. the guitar volume knob and first EQ pedal are both pre-gains for the distortion pedal, which is itself a pre-gain for the preamp stage of your amp.
The theory is that you can feed different parts of the EQ spectrum to suit the distortion characteristics of each.
In practice you can play around with all of this until you find what makes you smile.
This is something similar I found by quickly googling, but its not the site I am thinking of. That had a black-and-white drawing in it.
http://www.amptone.com/2eqswithpowerattenuator.htm
