#2
It means that the preamp - the part of the amplification process that produces and shapes the tone of your electric guitar - uses low-power "preamp tubes" to do so. In most guitars it's an 12A_7 or an ECC8_.
Hi, I'm Peter
#4
No. That's just a portion of the amp. I'm guessing you're asking about the Valvetronix series, which are NOT tube amps- they're hybrid. Which is to say they sound pretty much like solid state, not tube amps.
#6
There are all tube preamps (Mesa Boogie Quad), but it doesn't make up an entire tube amp. You also need the power amp section and a cab. If you're using a tube preamp, you'd want to pair it up with a tube power amp, that'll really produce the 'balls' in your sound.