Ah, ok. Well, "a cab is a cab. heads are the only major difference in sound" is what got me off.
Best way to compare cabs, is to piss the hell out of the guys at the music store and crank amps though them. Most any cab will sound decent at low volumes.(although, you can sometimes tell an immediate difference in sound, like comparing a Peavey XXX cabinet to a 5150 cab, the XXX is notably clearer and brighter than the 5150, even at low volumes. But, the 5150 cab's dark nature balances very well with the 5150 head. A XXX would probably be a bit harsh with that amp, but thats just pure speculation, I've never actually tested it, it could sound pretty good. But how a cabinet can handle a raging tube amp easily displays which cabs can cut it, and which ones just dont take it well. IIRC, Line6 uses Celestions in their cabs. Like I said, Not all Celestions are good, usually "endorsed" celestions, such as the ones in Line6, some Crates, and Fenders are just there for the name to boost the appeal of the amp, since "Celestion" is such a good catch-phrase and people who dont know the whole story will say "HEY, thats what my hero uses in his Marshalls!" and yup . .marketing 101 in action. They usually aren't that good of speakers. Its kind of like those "Duncan Designed" pickups, they say Duncan on them, but they are NOTHING like a real Duncan pickup. They're usually made really cheaply.
Another thing to take into consideration, is how you may want to choose speakers for your particular amp. While a color-free sound is ideal, your amp may have some piercing high end that cant be dialed out (pretty common for amps to be rather abrasive at concert volumes). For that, I'd balance it out with a warm and fat sounding speaker like a Vintage 30, or maybe even try out a 5150 cab with it to darken them up. Peavey cabs, construction-wise, are constructed very well. I have a XXX cab myself, and Its VERY good. The bedroom volumes arent as clear as a Greenback or something, but high volumes are handled REALLY well. Its a very tight, clear sounding amp.
The first thing I noticed when I tossed my Marshall 1960B cab for my XXX cab was an immediate "tightness" that was not there with the Marshall. I could play Scarified (a fast speed metal song by Paul Gilbert of Racer X, played on the end of the low E strings. . . a good staple song when testing out the low-end capabilities of an amp

) and all the notes would be excellently defined and seperate. With the marshall, it was almost like "woofwoofwoofwoofwoofofofofofofwoof." But with the XXX cab, it was like "chugachugachugachugachugadodadeechuga" . . .haha. . .but the point is, it sounded like NOTES! I could actually hear my string, not just the note it was making. Thats pretty impressive.
You know what sits on top of my XXX cab? a Laney 100w Pro Tube Lead 100w AOR tube amp.

Made sometime around 1985. . .has a great marshall EL34 crunch, but with LOTS of low end, and a meaner "growl" than a marshall. More musical and detailed high end too. Lovely amp, Its a keeper.