I think the PSX versions of my two favorite X3 stages are fine (Acid Seahorse, Gravity Beetle)
That's rather odd you pick those two... because those are my top two favorites as well
Actually, though, I do have a question for you Ash. Having never played any of the content in this box that isn't on the X1~6 box, is this box worth upgrading for? Obviously X7 and X8 are rare and expensive now, but is the music worth it?
When it comes to Mega Man X7 I have to agree with Goldfish even though two of the three tracks he posted (Cyber Geometry and Circuit Bomb) drive me crazy... though there are worse offenders. In a certain sense of the word I feel the Mega Man X7 soundtrack is a mess... but it's probably the best element of the game at the same time. As Goldfish mentions you can definitely cherry pick with this album - which is just not what you want with a Rockman soundtrack - and I think it's because so many (too many) composers were involved, which is ironic given how many composers worked on the original game. However, the difference is the style(s) the composers chose in the first game were cohesive with one another... this isn't the case with X7 where everyone wants to do their own thing.
In general I agree with what GX says about X8 as well, but I'd like to add a wrinkle that I've noticed with this one and makes some of the material on it a little less attractive. In most Mega Man / Mega Man X games the music is right at the forefront of a given level... this isn't quite the case with X8. In many of the levels in the game - Metal Valley, Central White and Pitch Black - the music is pushed so far into the background that it's is nowhere as engaging as it should be. The Metal Valley tracks are the worst in-game as all the commotion in the actual level drowns these tracks completely out and you can never forge a real connection with them. This isn't to say there aren't levels were the music plays an important part - Inferno and Jakob GX mentioned are great examples - but the fact that this situation exists at all creates a massive amount of conflict for the score in general. Because of that I can't give it a real recommendation even though it is more in the vein of the older games in some ways.
Command Mission is a weird one for me because I'm not really an Okada fan... but since this game was an RPG I didn't have any certain musical expectations for it so Shinya kind of got lucky with me liking this one. A lot of the music is too short and shallow but DO NOT underestimate the characters themes for the game's original characters... they are strangely one of the few things that makes those characters memorable.
As for Maverick Hunter X the only reason this soundtrack manages to come through its remixing is due to the strength of the original material. This becomes rather obvious once you hear an original piece like "Vile Stage Theme."
Additionally Xtreme and Xtreme 2 really aren't worth buying the set for as you'd expect. They are competently programmed so they don't really annoy but they're not going to beat out the SNES originals.
In the end I can only really recommend the Sound Box to the most devout Rockman X fans. To be honest... and from what you're implying... I think the Rockman X1-6 will suffice for you and most listeners. X1~6 is definitely the stronger half of the entire body of music. The various fixes are nice and all - and when you think about it the box is a really good value - but what was added over the last box easily is the weaker half of the deal.