The only "preamp" I see here is the volume potentiometer.
You can eliminate that.
If you take what you think is the preamp section out, you are left with something like this:
Do you have balanced drive - inverted and non-inverted, at the proper voltage swings to apply to the output tubes?
Leave it alone if you don't know what you are doing.
Set the volume of the Yarland to an appropriate level to work with the input from the other preamp and be happy.
Use the other preamp for volume control.
To be honest, I have no idea how to read that schematic nor have the skills to perform any DIY work on this amp. I was just trying to understand what mod is needed so I can ask a local shop to make the required changes and allow me to use this Yarland just for power amp and bypassing the integrated preamp as I already have a nice tube pre. Perhaps not worth the trouble from what you are saying.