It's interesting that this thread is so popular.
Let me ask a related question. Who has officially defined exactly what guitar tone is, and what is good guitar tone and what is bad guitar tone? Is there a law or a commandment or anything that says a guitar has to be amplified with tubes?
Another question. If I lined up the world's finest guitarists and plugged each into a POD and the recorded them straight to disk, would those recordings automatically suck? Satriani or Romeo would play and sound worse if they recorded with a POD? I really doubt it.
The OP wanted to learn how to get professional sounding guitar tones. I think there are 3 key ingredients that will deliver professional sound.
1. Have impressive talent. If someone sucks at the guitar, I doubt it can be fixed. It is still impossible to shine a turd.
2. Have decent equipment and know how to work with that equipment. This includes other key sound ingredients that havn't been touched on yet, like the guitar. A $50 made in china guitar with action in the centimeter range, inferior electronics and poor craftsmanship leading to poor tonality and intonation probably isn't going to give stellar recordings.
3. Good recording equipment. If one wants to record direct with a POD, Tonelab, V-AMP, GNX GT8 or even GR2 they will be good enough for a professional sound if the first two criteria are satisfied. If not, a good recording room, a good mic and a well maintained amp, solid state or tube, will be needed.
I must repeat something I said earlier. The listener really doesn't care if a CD is recorded with a POD or a boutique amp using the most expensive mics in the most expensive studio. They are judging your talent, not your guitar tone. If not, Malmsteen would never have been famous