"the perfect is the enemy of the good" -- so very very true. Why mess with it indeed?
On the other hand, I just had a peek in the back of my '69, and it looks like the baffle's just held in by 4 screws, no visible glue. In theory it should be fairly easy to install a 10" kit.
Speaking of the JBL K110 and Weber California, I sometimes run my Champ through both/either by rigging a wire from the RCA jack on the Champ to the K110 and alnico Cali mounted in my Princeton Chorus (I know, a waste for a SS amp). The Champ sounds even more fantastic through either, and has none of the shrillness of the PC.
Like you said, the size of the Champ suggests diminishing returns. But why not use a small extension cab? Then you can play around with speakers to your heart's content. I still do that, but that doesn't stop me wanting a good in-box speaker as well.
I'm curious about your Weber Vintage 8". Which one was it exactly? The 8A125T seems promising. The main thing for me is top end: the voxier the better.
Well, I have more time today to put the Champ through its paces, and my opinion has evolved a bit.
The Champ does sound pretty great in its current configuration. According to the sticker on the spider, the current speaker is actually Weber CVC8 -- part of the Chicago Vintage series. It isn't a very expensive speaker, but it does sound good, and it explains why it has such a great, bluesy breakup.
Although I don't have another 8 or 10 on hand to try, I do have both a 1x12 K120 (in a SSP cab) and a Peavey Mace 2x12 with K120s (don't laugh -- it actually sounds pretty good in a Southern-fried, Marshall-meets-Fender way, plus I think if I turn it all the way up you can hear it from space, lol). So I have some good speakers to play with.
Anyway, I plugged the 2X12 K120s from the Mace into the Champ (they are wired to a 4 ohm load), and it sounded good but was underpowered, which you'd expect -- enough power to hear them, but not enough to put the speakers through their paces. But it sounded good. I then plugged the 1x12 K120 into the Champ, and even though it was an 8 ohm speaker and a 4 ohm output, it sounded really, really good
. Much bigger and clearer, not underpowered, and just great. Immediately going back to the Champ's 8 was also revealing, as that now sounded a lot smaller by comparison. Makes it very tempting to change the output transformer to one that supports both 4 ohm and 8 ohm outputs, something I may do down the road.
I know I could get an extension cab and do it that way, or use my existing SSP cab, but the main reason I got this little guy was to have a small, easily portable combo amp. I have bigger amps that also sound good...but they are too big to haul to a friend's house and setup in his living room or garage without getting the stinkeye from his wife, making half a dozen trips back and forth to the car, and generally making a huge production out of things. Plus they they need some decent volume to really sound good.
So I'm still weighing my options. I like the Champ form factor, I just want to get the most from it I can. Changing baffles would be the easiest way to do this, though there's obviously the risk of messing up an otherwise near-pristine BF amp. Another option would be to get a larger 1x10 or 1x12 Champ replacement cab, install the BF chassis and a bigger speaker, and then storing the original BF cab. It's more money, but I think this option presents the least risk, and it's one I'm going to consider once I rebuild my gear piggy bank.
In the mean time, I may go ahead and order a 10" baffle and weigh my choices for 10" speakers. I love the K120s, but I've heard the K110s are a bit more finicky and can get rather ice-picky. I've heard good things about the Weber Californias, as well as the Weber 10A150 and 10A125. I've also heard good things about the Jensen Neo and Tornado Jet. And the the Tone Tubby's, too (though I'm not sure which TT model would be the right one to consider).
Thanks for your comments and suggestions!