Anywhere you put it, it will throw off your volume pots taper due to the fact that your volume pot is already a voltage divider. It will also attenuate your signal no matter where you put it. That being said, the only reason you need it is because at full volume the 1M is to high of a grid resistance. Even with another 1M right off the pot itself, that will give you 500k at full volume plus the grid stoppers resistance. This might might work though. You can put it anywhere you want. The main reason for this is to keep the grid, DC referenced to ground, and stabilize the grid current. Try it where it’s easiest for you though and see how it works. You never know till you try. The problem with the schematic is it was made with an EL84 in mind. EL84s had a max grid resistance of 1M. People have tried to put larger tubes using this same schematic, but they did’nt account for the lower max grid resistance of 6L6 or KT88.