Amateur answer: the lower B+ voltage has a number of effects:
- the input signal can not be as big;
- the voltage swing over the plate resistor, which is what you're looking for, can not be more because there is less voltage drop over this resistor;
- so the overall gain will definitely not be more, as a rule it will be less;
- there will not soon be enough 'drive' for the output tubes to give full swing.
If you really want to 'see' this, look up the tube characteristics for a preamp tube:
http://tdsl.duncanamps.com/link.php?target=3A259DCB
look on page 3, graph 'plate characteristics'.
This tells you for the different biasing, what the plate current is as a result of the voltage over the tube. As a result of the varying voltage on the grid (input signal) you will go from graph line to graph line. Take for instance -1.0V bias and 100V plate voltage, your plate current will be 0.5mA, not a lot. Assuming a 100kOhms plate resistor, the voltage drop over this resistor will be 50V, so you need a B+ voltage of 151V. Now a swing of the input voltage by -0.5V will change a number of things at once:
- you will go to the next graph line, which will try to change your plate current to 0.05mA;
- this current drop also occurs on the plate resistor, so the new plate resistor voltage drop will be 5V;
- since B+ is constant, this will push up the plate voltage;
- this will make sure that the current never becomes as low as 0.05mA.
The new equilibrium can graphically be found by drawing a line down to the right and up to the left from the point where we started. The angle of this line is determined by our plate resistor, so the line is called a plate load line. The voltage drop over the resistor is linear with the current through it, so it's easy to calculate the angle if you look at the axes of the graph (voltage and current).
Lowering the B+ pushes your load line to the left and it will go down. You can not 'jump' so many graph lines so your input signal cannot be too big (at the risk of getting harsh clipping). Etc: you can now see for yourself.
The maximum plate voltage is also listed in the data: 300V for this tube. This means that you can still get quite a bit more as your B+ depending on your plate resistor, and your output swing is going to be a lot more.
As for the 'drive' for your output tubes: these are optimised to give more power and gain is sacrificed. To reach full output power you want the thing to swing all the way between B+ and 0V. To do this with relatively low gain (say, 8x) you need rather a lot of input voltage swing, 30V easily. This cannot be done from the 12AX7 in the example above.
