Somewhere around 1k2 to 1k5 is pretty typical, this gives about -1V bias at a section current of a bit under a milliamp (two sections on a single cathode resistor, as it was, should be around half).
You will find a table of typical values here;
http://www.ozvalveamps.org/rcatriodes.htm270 ohms for the 6V6 looks about right. This is generally a 5 watt wirewound ceramic power resistor 'coz it gets hot - so don't cuddle it up to the cathode bypass electrolytic, they don't like heat. Measure the voltage across this resistor and apply Ohms Law to find the current, then multiply by the supply voltage to obtain the power - this should be less than the maximum anode power rating from the data sheet. {this simple method isn't
exactly accurate, but the error is small and on the safe side, it slightly over-estimates the anode power}.
Would I be right in saying that if I bias the Triodes so they don't clip as easily then they'd be pushing out (letting out) more of the original signal so would be driving the Output stage harder?
Normally we bias an active device (valve/transistor/FET) so that the output element (anode) has the largest available swing above and below its resting or quiescent point. This is the voltage that results in the output clipping against the HT supply rail (device off) and roughly ground (device saturated on) at the same time as the input is increased. This gives the stage maximum "headroom". If the resting anode voltage is higher or lower it will clip, run out of available voltage swing, against either the HT or ground first, wasting available swing on the other side, and reducing headroom to stage overload.
This is for "normal" audio applications, but for guitar some clipping on one side before the other may actually sound better than the Hi-Fi optimum, however the proper mid-point is a good place to start any experiments.
Note that while the device off clip against the HT rail is pretty well defined, the on clip at device saturation is less well defined with valves since they will still have some voltage across them even when saturated on - transistors on the other hand can go from off to hard saturated on, still a little soggy but much harder. For this reason the optimum resting anode voltage in a valve stage is generally a little bit above half the HT supply, a few volts.
{Voltage amplifiers like this don't so much "push" or "let" out their signal, so much as they vary the voltage on the anode up and down and the following stage "senses" this change (normally via a DC blocking/AC coupling capacitor). You can think of the classic triode stage as a fixed anode resistor in series with an electronically variable pot producing a rapidly variable voltage divider across the supply, the bias only required to make an imperfect real world device work properly.}
HTH
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