I think you mixed up C58 and R80.
C58 is a capacitor and R80 a resistor.
Put them back the way they were, and start experimenting with gain by raising preamp tube plate resistors (R16) or lowering cathode rasistor (R14) and play with its bypass cap values.
If you have removed cap C58 and now have only resistors there than I would say that the squell could be coming from there at some gain settings on gain pot VR3 and also because you removed C75/C76 but I am not sure as I am not familiar with DSL's...
I am sure some of the more experienced can give you more advice.