I ended up replacing the stock speaker with an Emminence 1518 Legend.  The speaker really opens up this amplifier.  I highly recommend this speaker for this amp.  I replaced the cheesy speaker jack with a Neutrik connector.  This speaker is big - a much bigger magnet than the stock unit;  I relocated the romex between the power section and the preamp.  Also, fitting the chassis back into the amp with this speaker requires some creativity.

The schematic for the modifications to the tone  section is shown below:

The Lite/Dark switch is a dumb idea.  I changed it to a variable tone control by inserting a 1M audio taper pot, jumpering R9 and R29, changing C19 to 0.02 µF, changing C2 to 120 pF, and adding a push-pull switch to the Tone pot which bypasses the tone stack.  Results are a significant improvement over the stock configuration.

Fender Excelsior

Fender Excelsior Mods

For Sale:     $420

Excelsior mods
Excelsior Mods
Fender Excelsior mods

This is an outstanding amplifier right out of the box.  Fender definitely got this one right! 

Two 6V6, 13 Watts, with a 15 inch speaker.  Instrument, Accordion, and Microphone inputs.  How cool is that?

No longer available, which is really too bad, it's a great, inexpensive amp.

I added a tone control and changed the LED power light to an incandescent bulb.  (Just because, that's why!)

I had a JBL D130F on the shelf, so of course I tried it out in the Excelsior.  The speaker is not a good fit for the amp.  It sucks gain and is clearly not matched.  In comparison, the Fender "special design" 15" speaker is actually very good.

I don't know why Fender stopped making these;  if I were to guess, I would say that they cost too much to build and there wasn't enough profit in it. 

If you can find one, buy it.  That's my advice.

Excelsior mods