[G6Tanuki]

The single-crystal-and-phasing-control at 455KHz was sort-of OK for CW reception but as noted in similar recent threads it's not that appropriate for SSB or DSB reception due to its poor shape-factor.

The balanced 2-crystal approach [replace the phasing capacitor with a second crystal spaced a couple of kHz from the first] works much better for AM/SSB. Tweaking the loaded-Q of the tuned-circuit the dual-crystal filter so it's mis-terminated works into gives you the opportunity to manipulate the practical bandwidth as also noted.

Q-multipliers have their place, but the disadvantage of using one to sharpen-up the bandwidth of a SSB/AM receiver for receiving CW is that a single high-Q circuit is prone to 'ringing'. Actually that's an issue with multiple high-Q circuits too...

Filters are a funny bunch, most people think of them as linear devices - under strong-signal conditions they can be anything but! A detail that seemed to escape some of the early designers of broadband front-ends where the first xtal filter could cause significant intermodulation if there was a bunch of strong signals being presented a long way outside the passband.

Also, there's much to be said for 'distributed' filtering; putting all your selectivity in one excellent filter at the front-end of the receiver, then following it with 100dB of wideband gain is silly.... those gain-stages will add wideband noise which intermodulates with the signal at the detector. Some supposedly smart radio-manufacturers didn't understand this [Icom, I'm looking at you!] and some of their radios were known for a high level of background noise. Putting a less-tight-than-the-front-end filter immediately before the detector to cut out the noise-sidebands is the answer here.