Are the Subpoenas Valid?

I am a long time registered Republican.

BUT I’m still very angry at the Republican defense in the Impeachment.

How dare they say that the subpoenas sent to the President were invalid?

I did some research. There is no legal statute defining what it takes to issue a subpoena. However, the Supreme Court has made several decisions stating that Congress may definitely issue and enforce subpoenas. They have left it up to the House to write its own regulations about how to do it.

When questioned about the validity of its subpoenas, the House passed a resolution (H. Res 660) on October 31, 2019. In this resolution they clearly stated that the various committees did have the authorization to issues subpoenas for testimony relating to the impeachment of the President. Previously, the House tried to enact a statute (HB 4010) with the intent to enhance compliance with congressional subpoenas. It was passed unanimously on October 24, 2017. But, this bill never made it into law because the Senate ignored it. The house has never taken up the question again.

The Republicans say that the House didn't follow the rules. They say the subpoenas are invalid and didn’t follow procedure. But with no legal statute, what exactly are "the procedures"? Are we to believe that the House didn't have any rules? What about the bill that they passed unanimously? What about their resolution to support committee subpoenas? Hasn’t the Supreme Court specifically stated that House committees do have the power to issue and enforce subpoenas?

Obviously, the President cannot be compelled to testify against himself and thus could refuse any subpoenas directed at him. However, subpoenas directed to others should be valid and enforceable.

This whole Republican approach to defending the President is ridiculous!