Selectivity enhancements

[talen]

I'd like to see some views on "selectivity" in these old tube
receivers and tradeoffs for the various methods from those
who have been there, especially with homebrew equipment.
Bandwidth data, insertion loss data, effectiveness, retrofit
difficulty, cost, etc...... if available, would be very interesting.

For example.........
* Iron core vs Air core IFs (higher "Q" with iron core)
* Multiple IF transformers...several 50s articles had series IFs
* Lower frequency IFs....going from 455Khz to 100/50Khz IFs
* Various Crystal filters to enhance and null signals....lots of
    articles in the 50s and 60s on this because it's cheap
    and adaptable to existing receivers (had one on my S40B).
* Q-multipliers, Select-O-Jects, etc.
* Mechanical filters
* and ???

73s  Kees K5BCQ