Actually on the Champ12 both the headphones and the line out are after the OPT.  IMHO the design defects you are hearing are everywhere else in the circuit ;)

The line out can do a good job of capturing the small Champ12 sound for further amplification by a PA or something, the speaker is still running and is part of the equation, I replaced my original Fender (Eminence) speaker with an 8ohm Celestion and it improved the raw sound of the amp as well as it's line out performance.

When you plug in the headphones it does shut off the speaker and replace it with a resistive dummy load, so you lose the reactive speaker interaction, this way the sound is further lacking.

I worked on mine a lot and IIRC replaced over 20 components with higher quality parts and many of different values to voice it more for Fender
& Gibson guitars to my ears, who knows what they were thinking when they let this one out of the factory like it was.

One of the most dramatic improvements to me was to fix the power supply so that the screen of the 6L6 was much closer to the plate voltage,
the factory setup with low screen voltage really was wimpy, but maybe it was intentional to keep the temperature down. It does have a good
OPT for Class A single 6L6GC use with an 8ohm load, too bad they put it inside the chassis (near the reverb *tank*), it gets too hot when you
run the 6L6 at what I consider more reasonable Class A dissipation now. So I always run a small desk fan behind the amp, that way it runs
cooler than a regular Champ. Well, one of the helpful things also was to replace the old 6L6GC with a NOS JAN 7581A which can take the heat
just fine and really rocks so naturally I pushed it to the max.
When I get another amp bench set up, and my notes sorted out, if there is any interest I'll make an outline of my mods for the Champ 12.
Hi fred,

I worked extensively on my Champ12, and thought your message was familiar just by reading the title. This is a screwed up amp design from the factory, I put it in the same category as other products with design defects that are obvious upon initial inspection, and where more continue to appear with further investigation.

You would think a SE6L6GC & 12inch speaker would be louder than a SF Champ & its 8inch speaker, but no . . . until you stomp the overdrive
button, then its lots louder, not enough to require earplugs for hearing protection, mainly just to save your ears until you're playing an amp with at least marginally musical overdrive ;)

There are some bad points about this amp too . . .

500V B+ and 250V screen were intentional, what were they thinking? If they were thinking, it was probably about compromising the things I
value most in a Champ.

When I worked on the power supply and got the screen closer to the plate voltage, it made my guitar sound a little more recognizable. I did not lower the plate, I increased the screen, and the plate declined slightly as a result.

This also brought the 6L6 operating point more into the area where Class A operation was possible without sacrificing most of the power the 6L6 is capable of.

Naturally with the dismal nature of this design, the other hardware is not fully capable of supporting a tube as big as a 6L6 if you are trying to get nominal performance from an NOS tube. I upgraded the resistors that were unsoldering themselves, plus tweaked the power section and fixed the *designed to hum* grounding problems, and basically felt I had done all I could with the 6L6 and the amp was still a dog.

So I had to get into the preamp/driver section, first replacing key sub-standard quality parts with the original values using acceptable
components, then auditioning a huge improvement before tweaking some values to work somewhat musically when you are playing the guitar.
The clean section was challenging enough, and I touched on the overdrive to remove the repellent sound but did not want to spend the time it would take to actually tweak it into an attractive overdrive. I've now built a transistorized and a tube overdrive pedal for my guitar where either one drives the clean Champ12 into much better overdrive than the stock *waste of a 12AX7* internal circuit.

Removed 90 percent of the hum, so now it only hums 10 times as bad as a BF champ. I feel the reverb does need to be relocated from inside the
chassis, might even benefit from a metal reverb tank instead of cardboard, and it could probably benefit from shielded reverb cables. Didn't want
to do that at the time either, so I decided to button it up for a few years before completing the mod. Eventually I'll probably gut it and wire a point-to-point 6L6 vibrochamp into this platform, but first I want to complete the mod on the PCB so I can have a tested board to trade to someone who might want to upgrade without all the hassle. So far there are about 25 significant changes in dozens of hours of bench sessions.
Looks like you've started on your preamp section already, I took the opposite approach and wanted to get the most out of the 6L6 before I
voiced the preamp to compliment it.

Back in the cabinet, the SE6L6 does run too hot for the PT, and I'm OK with the OPT inside the chassis WRT noise (if its done right, Champ12 is not TOO bad), but it contains the heat too much, so that's the big problem with trying to get nominal performance from a SE6L6. I made it so this amp can BARELY handle playing in 90 Fahrenheit room temperatures without a fan, but I still always use a desk fan on high blowing at the 6L6 and the chassis in the area of the PT even when the room is below 70 degrees. This is better (cheaper, quieter and more easily replaced at WalMart) than a muffin fan would be, it also ventilates the entire open back cab.

The 6L6 now runs almost as hot as I would in a non-compromised design, and I think the JAN or STR US NOS are definitely the thing to stick
with. The original Fender tube is just fine, a NOS JAN Philips 6L6GC works just the same and has only a slight benefit being new out of the box compared to the old original tube. Heck, the old tube was loafing along for years, it never did do much work until I got a hold of it :)

Even with a fairly efficient Celestion 12inch speaker its still not as loud as a BF Champ, I'm not a miracle worker . . .

Hope this helps,
Mike