I believe those are input protection diodes. They'll limit any incoming voltage to about 1.2V peak to peak to prevent damage to the rest of the preamp in the event of someone hooking up something silly (within reason). They'll have no effect on sound unless your guitar is cranked up to super hot and you're picking like a fiend (though this depends a lot on the pickups), and then you'll hear the sound break up during the peaks. If you add those two diodes and find you can't crank things up like you want without clipping you can add another pair in series with them and that will double their effective range from 1.2VPP to 2.4VPP. If you're putting out that hot a signal though you'll find you have less room on the overdrive knob before it gets really dirty.
The short version: they are safe to omit if you're careful about what you plug in, though you might want to experiment with them just to see if you like what they do.
Hope that helps.
The short version: they are safe to omit if you're careful about what you plug in, though you might want to experiment with them just to see if you like what they do.
Hope that helps.