Can you use a Pass INT 250 as an amp (bypass its pre amp)?

Yes,has preamp inputs.One set of rca single ended and one set Xlr balanced.
I use to own one of these Intergrated amps and it works great with another preamp of your choice.

No there is no “main in”
You would simply treat your external preamp as any other source to the 4 inputs; set the volume control on the pass to about 55 and use your other pre;
the internal line stage in the Pass is very transparent and will let the character of whatever preamp you use to shine through; 

you can use the PREAMP OUTPUT  to drive another amp or sub 

I have this Int 250 and really like it.

You have to spend double for separates of equal power to improve; good luck

Oops,
dpac996 is correct and after I read the manual it confirms there is no dedicated main preamp inputs.
it’s been 5 yrs since I owned one of these,with lots of different gear in between.I currently have a new Modwright and it definitely has them with a HT bypass.For now I’m just using it as Intergrated but I’m sure over time I will experiment.