Preamp and impedance question

I am asking a custom builder to build an amp using Luxman mq-300 schematic.  I have two questions:

1) the Mq-300 is a power amp.  I don’t have a preamp.  Can I just add a volume pot to a power amp schematic?  I did some research about passive preamp, but it seems even a passive preamp has a circuit and a transformer.  Is it better to buy a separate passive preamp?

2) the transformer is designed for 8 ohm.  If I want it to support 4 ohm, I need to upgrade the transformer.  I know that the speaker impedance curve can dip to 5 ohm at 30Hz.  Is it important to have 4 ohm support?

thanks.