OK Herman, yeah maybe a little, or maybe more than that :) But really, its pretty easy. If you drive the cathode of the input tube rather than the grid you have what is called a grounded-grid circuit. You need a coupling cap to do it. Turns out that the value of that cap is the same if that cap were to be used as a cathode bypass cap, in the case of normally driving the grid.
So, why not drive both the grid and the cathode in opposition? In practice, this works quite easily. You install an XLR on the amp, and attach the cathode bypass cap (if there is one, if not you have to supply one) to pin 3 of the XLR rather than ground. Pin 1 of the XLR goes to ground, and pin 2 goes to the RCA input. Now if you drive the grid only, single-ended, you will want to put a jumper between pin 1 and 3 if the cap was a bypass cap. If not, you don't need the jumper.
The only trick is that the grounded-grid input is pretty low impedance. You might want to have an auxiliary resistor that plugs into the RCA input that sets the normal input (pin 2) to the same value. This is not a problem if your preamp supports the balanced line standard.
I have this hookup running right now on a set of DIY amps I built, which are a set of push-pull amps using 45s for output tubes (class A), and a single-ended driver circuit similar to a Dyna ST-70, only all-triode. I didn't bother with the extra resistor, as this input behaves a lot like a differential input. I guarantee you that if you tie the two inputs together and try to inject a signal, you will find there is a substantial CMRR.
