Even if you could lower the gain in your MM phono stage with a 12AT7 tube swap (which is not advisable anyways, as others have mentioned), it wouldn't help your hissssss noise floor much because the phono stage is before the preamp's volume control - so what you REALLY need to improve is the signal-to-noise ratio, rather than any absolute noise floor.
Your tools for increasing signal-to-noise ratio in your vinyl source ecosystem are as follows:
1. Get a different cartridge with higher output level. Your Dynavector's 0.3mV output is right around where these things start getting difficult.
2. Use a SUT with a slightly high ratio - you're already doing this, and 20x is a good ratio for your cartridge. You could go even higher (say 30x) to further increase signal-to-noise, but then the flip side is you start cutting closer into the MM stage's overload margins (i.e. higher distortion).
3. Don't load the cartridge too heavily, or you will lose signal, while noise floor stays the same - hence a loss of signal-to-noise ratio. With your 20x SUT, the 47K MM input, and a 5-ohm cartridge coil, you should be OK here.
4. Get a higher quality, lower noise MM stage. Easy win, if you don't mind spending the money ;)
5. Source as low-noise tubes as you can, when your MM stage is tubed. You may or may not already be near your stage's low-noise limits, with the stock tubes - only one way to find out! But in Chinese preamp w/ onboard phono, I wouldn't be surprised if they didn't take any care selecting the stock phono stage tubes. Even the better Chinese brands are notorious for shipping with crappy tubes.
Your tools for increasing signal-to-noise ratio in your vinyl source ecosystem are as follows:
1. Get a different cartridge with higher output level. Your Dynavector's 0.3mV output is right around where these things start getting difficult.
2. Use a SUT with a slightly high ratio - you're already doing this, and 20x is a good ratio for your cartridge. You could go even higher (say 30x) to further increase signal-to-noise, but then the flip side is you start cutting closer into the MM stage's overload margins (i.e. higher distortion).
3. Don't load the cartridge too heavily, or you will lose signal, while noise floor stays the same - hence a loss of signal-to-noise ratio. With your 20x SUT, the 47K MM input, and a 5-ohm cartridge coil, you should be OK here.
4. Get a higher quality, lower noise MM stage. Easy win, if you don't mind spending the money ;)
5. Source as low-noise tubes as you can, when your MM stage is tubed. You may or may not already be near your stage's low-noise limits, with the stock tubes - only one way to find out! But in Chinese preamp w/ onboard phono, I wouldn't be surprised if they didn't take any care selecting the stock phono stage tubes. Even the better Chinese brands are notorious for shipping with crappy tubes.