Learn to use DSO

Hi, i am thinking to buy a DSO Quad for my diy tube amp project. So i like to know whether
this digital scope can be used to measure my tube power amp with 450VDC B+?

450 V? no , the DSO do not support such a high voltage.

How large of a voltage can you measure using a 10x probe?

400v,with X10 probe, which is not included in the package of Quad .