It's not a very good idea..
The G9 output is driven by an ECC82 SRPP stage, with a Zout of around 2400 Ohms.
So if you load it with a 1:1 transformer, your resulting output resistance will be 2K4 Ohms - a bit too high - and your output level would be 6dB higher.
But in order for this to work, you would need an output transformer that will work well (ie pass low frequencies) with a source Z of 2400 Ohms. These are not common at all.
The 2:1 output transformer ensures an output impedance of 600 Ohms, and I don't think that you'll need higher output levels than this delivers already.
But on the other hand, if you need more gain (e.g. using weak-output ribbon mikes) it would better to experiment with ECC81, 12AY7 or ECC83 for the first gain stage. They should be more or less plug-in compatible.
Jakob E.