Question about Preamplifiers and Power Amps derived from complete amplifiers

[kagliostro]

Assuming the intention is to derive some preamplifier from schematic of existing amplifier, is to consider to "cut" the circuit right before PI and than consider the obtained preamp circuit complete

or is better to add one further tube ?

if is better to add a tube

is it  good practice to look to FX loop circuit and use the circuit of the tube used in the send circuit as add on ?

for the same reason a power amp can start from PI or is better to add the circuit as in the return of an FX loop schematic ?

I can plagiarize something, not invent it, so I need your suggestions and help

Thanks

Kagliostro