If you use normal rules on subpoenas...only the Senate committees have this power...not the broad senate itself. An example...the Senate Judiciary Committee could go and issue the subpoenas, but the only GOP guy I can see who’d go and vote on this is Graham...the other 11 GOP guys won’t agree with this. In a 11-11 situation...nothing happens.
Maybe Justice Roberts thinks he has this subpoena power.
If that is so, it is only because the senate as a whole delegated the power.
As for conduct in an impeachment trial, we already saw Schumer attempt to get 51 senators to agree to have subpoeans issued to certain people and institutions. This is an example of seeking "whole senate" approval for subpoena.