212HD130 power amp in?/outlet?

I want to use the tube output stage of my hd130 and bypass its (weak-sounding and too prone to distortion in lower registers) preamp. Where should I inject the signal and what amplitude (peak) should it be? Basically, where does the power amp start distorting in full power mode?
I'm thinking of making a true tube preamp so that i can get a decent overdriven (TUBE) tone that isn't 'fart fuzz'. (what would be ideal would be to be able to footswitch between the two for extremely-clean and mildly-distorted tones -- meaning a non-invasive mod)
I've got a 2475 chassis, and from what I can try to read of these schematics it looks like maybe off of pin 4 of the LM307 connected to point D... This then goes through the deep switch, past a few resistors to ground and to pin 7 of the 12AX7. Should I do it after the deep switch?
I'm really looking for a simple mod -- perhaps a normalled 1/4" jack for the preamp in mounted in the back somewhere. How does fender do this stuff? somewhere near EVERY ONE of their amps has that feature...
The power being pushed would still be low enough to allow me to use instrument cables, correct? (i don't know exactly what voltages it's expecting, so I don't know.)
Also, what's the deal with the built-in second outlet? I can just replace that with a better single outlet that doesn't allow the cord to shake loose and then use that to power my (yet-to-be-acquired) preamp, right?

You could install an interface point at the input to the power amp section that would allow you to drive the tube power stage from the output of a tube preamp.
Usually, you can simply install a normalized jack at the high-side of the Master volume control to accomplish this. Depending on the amp's design, you might have to provide a coupling capacitor but this probably wouldn't be necessary in most MM amps.
Different models might have different requirements. The RD and RD models already have this jack at the front panel (the 3V Insert jack). Check out the schematics for these models for a clue on where you might put this jack in your circuit.