[regenfreak]

I didn't mean the output of the IF strip, but the output of the IF transformer of front end, and assumed it having a smaller bandwidth than that of preceding stages. You could get a meaningful picture, improving the resolution of the sweep by adding another IF filter (of high Q) past this IF transformer.
And sorry, I had a typical transistor-oriented IF secondary impedance ~100-200 Ohm in my head.

As I said before, once you take the output of the first IF stage, you mask out the critical information about the tracking and sweep response of the variable double tuned stages across 88-108MHz. The IF stage has its own response shape. The sweep response from the output of the 1st IF stage does not show the true overall response of the single-double-double tuned stages of the RF front end. I dont have the proper equipment to measure distortion.

I managed to do RF sweep of RF frond end of a 4 gang FM double gate FET tuners with high Z probe but have not been successful with valve FM tuners. Great caution must be taken with valve tuners.

I am starting to build the dual gate FET version of the 6-gang tuner. It should be easier to test and align without worrying about the high voltage or frying the measurement equipment. But it will be still tricky to align those double tuned amp stages because we still have high impedance in the I/O of the filters.

the IF transformer bandwidth should be made considerably wider than the expected front bandwidth

The opposite is true. The IF stages provides lots of the selectivity. Typical FM IF bandwidth is about 250KHz for wide IF high fidelity stereo. The RF front end has quite wide bandwidth (see the RCA paper example in my previous post number 20 and 25 of this thread. With double-single tuned stages, it is about 700-800kHz). This is because it is very difficult to align multiple tuned gangs (over 3 gangs) because of the tracking errors and imperfect filter response shapes. The bandpass shape and bandwidth of the double tune filter changes with the frequencies across the band. 2nd order Butterworth bandpass filter has very sharp tuning; a small misalignment would cause massive insertion losses. On the other hand, a single tuned LC gang has a relatively low loaded Q, small misalignment wont hurt much.