To add to my above comments, it seems to me that since the Constitution establishes a tricameral government, expressly for the purpose of providing checks and balances, the Court could satisfy its duty to that end either by providing information through direct testimony upon request by Congress or by compliance with a subpoena issued by Congress.
While some constitutional Scholars maintain that Congress does not have the authority to subpoena members of the judicial branch, here is the law as it relates to the matter;
“According to Article 3, Congress has the plenary authority to cause every Federal court in the country with the exception of the Supreme Court to cease to exist. Further, as declared in Article 3, with the exception of a narrow class of cases, Federal courts would have no jurisdiction to hear most cases absent express authorization from Congress.”
In other words, lower courts serve at the pleasure of, or privilege provided by, Congress. Since only the Supreme Court is expressly removed from Congressional oversight, it seems members of the FISA court would have no choice but to comply.
And don’t forget, Article 3 also give Congress the Authority to Restrict what the supreme Court can hear.