this._http.get<JSON>(url, { params })

.pipe(map((res: JSON) => this.getObjectFromJson(res)))

.subscribe(myObject => {

    this.getValueToUpdate(myObject).subscribe(newValue => {

        myObject.some.value = newValue;

    });

});

// return Observable<myObect>;

this._http.get<JSON>(url, { params }).pipe(

  map((res: JSON) => this.getObjectFromJson(res)),

  switchMap(myObject => {

    return this.getValueToUpdate(myObject).pipe(

      // probably here you need also a catchError(err => ...),

      map(newValue => {

        myObject.some.value = newValue;

        return myObject;

      })

    );

  })

).subscribe(myObject => {

  // here myObject is updated with newValue

});