Okay - AGAIN at 0:27, I can see what looks like a blue metal film resistor swap to R13 to the right of that monster gumdrop cap.
That sets the gain at the op amp. Stock is 4.7K - most guys make it smaller.
The monster cap is the "treble" cap (C11), and increasing it adds midrange. Lots of modders increase it from the stock value of .022uF to something like .1uF, and that looks like the case here, or it could be .047uF.
...Just making R17 bigger has the same effect, and C11 can be left stock!
What makes more sense - replacing a 2 cent resistor or trying to cram in an 80 cent (or more) capacitor? At the very least, just do what analogman does, and put a little Panasonic .1uF cap in C11's spot...he's been modding 'em for years and knows daggone well that a big cap like that would pose a warranty issue (on the modder's part, if they offer one!).
But simply by swapping the LED and making the value of C11 bigger (not the PHYSICAL dimensions - sheesh!), you will get more mids, and much more output.
Edited - I can see that C3 (down near the "front") has been changed, too. That is a common swap to make the pedal either tighter or bigger sounding (but not really either - depends on if the cap value is made bigger or smaller).
It looks like only C3 and C11 are changed, so kudos for that. Many modders "shotgun out" all of those vertical black canister caps and replace them with film types like those monster Vishays. If anything, the "canisters" are supposed to be "the major source of a lack of amazing tone," since they are electrolytic types. They are all or mostly .47uF, and the common thing is to bump them up to 1uF.
So much for that having an effect, eh?
But what I see just goes back to what I tell folks to do if they want some more mileage from a stock DS-1 without going nuts:
- add more "threshold" at D4 & D5 (IMO, a LED @ D5 is "excessive," but to each their own)
- make the value of R17 bigger to get more mids (and leave C11 alone!)
- change R9 to 100 ohms, and you probably won't have to mess with things like swapping C3
...Stock value for R17 is 6.8K. Make it 10K for a modest mid bump, 15K for a strong one, or 22K for "Marshall madness"
MIJ clipping diodes had a forward voltage of around 950mV. MIT diodes are around 600mV. Add a germanium or Schottky diode (forward voltage of ~400mV) in series with both D4 and D5, and you get ~1000mV; same as MIJ.