an overdrive pedal basically will change your tone in some way, tubescreamers are notorious for altering the mid-range, for instance. A clean boost is 'clean' because it doesn't change the frequency of your guitar tone, as I think I mentioned earlier on. Both will give you extra volume and gain too - whether solid state or valve.
I don't know why it works with a solid state, but having tried it, it does. I got more gain, heaps more gain. I assume that this works along the same kind of principle that means that a hotter pickup will give you more gain, regardless of whether the amp is solid state or valve.
ah, ok, fair enough. Point ceded, lol.
I actually meant to put in about the tone changing with an OD, but i forgot, lol.
Also, what i meant about the OD getting its own gain was that the overdrive was actually coming from the pedal (EDIT: although with the gain/drive control on the pedal at minimum, and the volume/level up full, the OD will mostly be inducing overdrive from the amp too), whereas the clean boost was inducing overdrive from the amp (i'm confused now, lol).
Lol- sambo, they're kind of similar, but different, lol.
I was wrong about the SS amps, though.
Cheers,
:D