Interesting about how prog metal is a broad and deep genre given that last year you argued the opposite, stating that bands like Mastodon and Cynic aren't prog.
And I still feel that way. Nothing has changed. Mastodon's most recent output has little to nothing about it that I personally would label prog. I don't remember discussing Cynic to be honest. They are pretty technical, which certainly places them in the broader prog landscape imo. Tech thrash is the term most people use to describe their sound. To me, that is a pretty close cousin to prog music.
Prog music IS very broad and deep. I'd say there's a pretty broad range between bands like Riverside, Magic Pie, Fates Warning, Haken and Dream Theater. Wouldn't you agree? Yet all of them fall within the general scope of progressive music. And by the way, I don't necessarily LIKE all of those bands. But I do agree that they fall into the general category of progressive music.
It's pretty simple really. I believe prog has a certain level of musical complexity to it. Otherwise, it isn't really prog. When you go back to the 70's, to the bands who really pioneered this stuff, they stood apart from other rock bands because of the complexity and sophistication of their musical arrangements. That's how the term 'prog rock' was coined in the first place. Bands like Yes, ELP, etc stood out from other rock bands of their era because they played music that was on another level of technicality musically. Critics even dismissed their music as 'pretentious' and 'boring' and 'noodling' because of the long complex instrumental interludes. So people came up with a label which they felt was descriptive of that musical difference...progressive rock. I stand by that definition.
Others here, of course, disagree. And that is their right. But their arguments break down when you ask them to describe specifically what prog means to them. Many respond that a band is 'progressing' when they change up their sound from one disc to the next. If that were the criteria, then Metallica playing an unplugged disc would be 'prog'. Which is silly. At least that's how I see it.
When it comes to Symphony X, as I stated before, their sound has changed dramatically. For example, they have almost entirely dropped the neo-classical (think Yngwie) sound which was so prominent in their early material. That factor alone has greatly changed their sound...and turned off a number of former fans. But I still contend that they write songs which are highly technical musically. Just ask any guitar player and they will tell you that is a fiendishly complex riff he is playing in 'Nevermore'. Let's put it this way; if Cynic is prog then Symphony X certainly is.
All I'm really trying to say is that labels matter. You can't just label a band 'prog' because you like their sound, otherwise the term 'prog' loses all meaning as a descriptor of musical style. Like I said before, Rascal Flatts is NOT prog. You can slap a water label on a bottle of alcohol, but it won't change the content.