Yes, it is 100% stock. Even the tubes are stock.
I'll probably try new tubes soon, but I don't think I'll change the speaker (good speakers are very expensive here). As for the C19 mod, it sounds interesting, maybe I'll try someday, but for now I think it's fine like it is, I just roll off the tone knob of the guitar a little bit.
Just going over this huge thread, as I just purchased a DSL40 from GC, picking it up tomrw.
Exactly with respect to tone, watch this.