First of all, I had no idea how "budget" the stock speaker was until I removed it. It has a thin, ceramic magnet, and weighs almost nothing... It's like a stereo speaker. The Weber has a big, honkin' alnico magnet, and is much more heavy-duty in construction. This speaker has made the amp bigger, louder, and more responsive to playing dynamics, and the projection is fantastic! It's like going from a 2-D photo to a 3-D movie. This speaker really brings out the quality of the amp modeling, too. I thought the modeling on this amp sounded pretty damn good before, but now it really feels like I have a tweed Deluxe, or a Princeton Reverb in the room, with all the subtle nuances and characteristics of the genuine article(s). And OMG, the cleans are incredible! The highs are so smooth... With the old speaker it could get pretty piercing and harsh if you got carried away with the treble. With this speaker, I can dime the treble with no ice-pick or painful harmonic artifacts.
This speaker has made me fall in love with this amp all over again... When I'm at work, I keep thinking about it, and can't wait to get home to play! It has also made all of my guitars sound noticeably better, with richer harmonic content (yes, I've played every guitar I own through this amp since I replaced the speaker). I can't remember the last time I was this excited about a piece of gear! Especially a piece of gear that I already own! I expected the Weber to sound tighter and have generally better tone, but I didn't expect the difference to be this dramatic! I wish I had done this much sooner.
I also changed the tubes and biased the amp (there's an hilarious story about what led to that), switching from the stock Groove Tubes to JJs (for no other reason than I found a good deal on a set of JJs). I don't know how much that affected the tone, or general sound of the amp, because I replaced the speaker at the same time. Maybe some of you JJ fans can educate me on the differences?
There is one downside, though... I used to run this amp with the volume at around 4 or 5. That was loud enough for the amp to "bloom", but wasn't too loud for apartment playing. Now that this amp has more output, that point where the tone opens up is just over the cusp of apartment-friendly volume. I can't turn the volume up much past 3 before it gets too loud (with the gain at about 2 or 3), but it doesn't get to sounding really good until you get to 4 or higher, and that's just way too loud now. Usually I start with the volume on 3, but I'm always wanting to cheat the volume up by microscopic increments, trying to find that hair-line threshold between optimal tone and reasonable volume. But then I find I have to hold back too much in my playing, and I turn it back down to 3... Which doesn't sound bad, but you know it can sound better.
Overall, I don't regret the swap one bit, though. If you have a Super Champ (XD or X2) and you've gotten kinda complacent or bored with it, try upgrading the speaker. It's like getting a whole new amp... A whole new awesome amp!
), and put a better speaker in that amp. 
Yesterday I went out in the living room with my wireless transmitter, and the amp set on 3 to get a feel for how much of the sound is bleeding through the walls. It wasn't bad. You could definitely hear a low, muffled sound that would probably be rendered mostly inaudible if the TV were on, but I wouldn't want to push it much louder than that. I also don't enjoy playing on the threshold of "too loud". I have to restrain myself too much, and it affects my playing dynamics. The good news is; I did a lot of tweaking last night, and I figured out how to get that pleasing tone at 3 instead of 4... depending on the output of the guitar, that is. I also realized that now that I can hear more of the unique characteristics of each of my guitars, I'm going to be spending a lot more time working the EQ and other settings when I change from one guitar to another (which I do quite often). All of my guitars sound different from each other (what's the point in having a bunch of guitars that sound the same?), but now they sound even more different.
