Phil, usually the feedback resistor is the one connected between preamp output and the first line stage tube cathode.
Do not go overboard with feedback, it will kill soundstaging and transient response.
A good tweak to reduce gain that also improves sound quality is to remove any cathode bypass caps in the line stage.
To get rid of ringing (actually RF oscillation), install 300 ohms to 1 K ohms (not critical value) carbon composition resistors in series with each tube grid. Allen Bradley brand is one of the best for this application, available from Parts Connexion for the whopping price of 0.30 each!
These are called grid stopper resistors and MUST be installed very close to the tube grid socket pin. Cut the resistor lead to be soldered to the tube socket no longer than 3/16 " (or 4.5 mm for the rest of the world).
This tweak will damp only HF oscillation and will NOT harm audio performance, unless you overdo the resistor value (1K is the safe upper limit, 300 or 470 ohms should do it).
Also, scope the power supply output to make sure it is not ringing and has no rectifier spikes.