I'm still in the process of building my GG, so I can only confirm that it is cheap and fun. As far as good sounding, there are many reviews of this preamp on Audioreview and AudioAsylum. I would start there. Have fun.
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The GG has a DC filament with a voltage regulator. I eliminated the voltage regulator and used Schottkey diodes and a couple of resistors like snubbers to reduce voltage for the filaments.
I used UF4007 diodes with snubbers for the main supply.
The input specs a 10K grid stop resistor. I used a 68K tantalum resistor with an Alps pot in shunt. Used tantalum resistor for other resistor in signal circuit.
I tried Auricaps 3.3mfd output caps. It could possibly shatter glass with the Auricaps so I used Jensen copper foil PIO caps to tame it some.
I don't know about the extra output but I would imagine it is possible. I have only one in and one out. There is an on off switch and the volume control, that all.