First off, remember we're designing guitar amps, which are
SUPPOSED to add coloration to the sound--there is no "right
way" to do anything in this kind of application.

Having said that, here are some general pointers...

The inductance of BOTH of the transformers will play into the
ultimate low frequency response of the amp. Here is an
excerpt from Steve Bench:
------------------------
There is a formula for the low frequency part:

F = R/(2*pi*L)

The resistance in the above formula is the parallel of the 
primary side resistance load in parallel with tubes plate 
resistance. Notice that since the effective plate resistance
varies with the instantaneous operating point, the response will
also.

This gives you the 3 dB point. The 1 dB point will be one
octave (2x) higher, the 0.3 dB point will be one octave higher
still.

You can re-arrange the above formula to solve for the
required inductance:

L = R/(2*pi*F)

Example: A tube with about 1k plate resistance is operating
into a load of 3k. It is desired to have the lf response be down
1 dB at 15 Hz. What L is needed? Solution: Since the 1 dB
point is desired, the "pole" will be 1 octave lower at 7.5 Hz.
The R effective is 1k || 3k or 750 ohms. Therefore
L=750/(6.28*7.5) or about 16 Hy. Don't forget that the L
value is also the parallel combination of the (dry) transformer
inductance and the choke load, assuming the capacitor
coupling the two isn't close to resonance. [I am assuming a
parafeed arrangement from the nature of your question].
Soooo, if the transformer inductance is 32 Hy, you would
need a 32 Hy choke to achieve this response.

Notice that for higher resistances, more inductance is needed
to achieve the same lf performance.

On the high frequency point... that's not *directly* controlled
by the inductor value. It is controlled by the transformer's
capacitance and leakage inductance, the tubes output
capacitance, stray capacitance, and the parasitic capacitance
of the choke. Larger chokes tend to have more parasitic
capacitance, and thus more effect on higher frequencies,
although there's no 1:1 relationship as it is a parasitic effect.

On the issue of does the plate current affect the required
inductance:
twofold effect; the plate current will affect the effective plate
resistance of the tube, thus the response as given above.
Notice that higher currents generally produce lower plate
resistances, and thus lessen the needed inductance. However,
the inductance of *any* choke varies to some extent with
current; generally causing the effective inductance of the choke
to decrease with higher currents. Fortuitously, these two
effects tend to offset each other.