A few days ago I got the two Weber speakers I've been waiting on. Took exactly 14 business days for them to make them, which is what they say on their web site.
I wanted a brighter-toned speaker for the Champ 600, and I wanted an 8 inch speaker in the hope of more bass and volume. I bought a Weber "Blue Pup," which is similar in tone to the Celestion alnico blue "Bulldog" speakers used in early Vox AC-30s.
Here's the test fit with the new speaker and a thinner (1/4 inch) plywood baffle. It works!
I didn't document all of that - please use your imagination to picture a square of plywood with some screw holes and an 8 inch hole in the middle.
The thinner baffle board is key to getting a bigger speaker into this amp. Part of the speaker will go up under the chassis. It will not clear the chassis with the stock baffle.
You can see a tiny bit of cone is blocked by the cabinet - not a big deal. You can also see what I meant by the speaker going up under the chassis - the top of the speaker basket goes up into the space between the chassis and the cabinet at the top.
I reused the tweed oxblood-style grill cloth I had previously put on the old speaker baffle.
The magnet on the speaker isn't that big, but there might be clearance issues with an 8 inch Jensen-style alnico Weber speaker fitting into this cabinet. The 6 inch ones will work, but if you're thinking of trying an 8 inch, measure carefully!
The amp sounds pretty big now, and it's loud as all get-out. The high end is a little chimier - but the mids and bass are really something now. I really like it.