Replace the very wide 455KHz IF "roofing" and standard AM mode filters with something more appropriate. The "roof" filter (FL1501, a MuRata CFWS455G) is some 15KHz wide while the AM filter (FL1341, a MuRata CFWS455E) is 9KHz wide.
WARNING: If you have no experience with SMT parts removal and replacement, you will likely run into issues removing the filters as they are soldered into plated thru-holes, and close to other parts populating the board. Do not attempt this mod if you have the least doubt as to your soldering skills. Although none of these filters are SMT parts, they are a true pain(tm) to remove. Don't say I didn't warn you.
Two MuRata CFW-type 455KHz filters of desired bandwidths, soldering pencil, solder sucker/desoldering bulb, solder. The filters I used are CFW455HT (7KHz nominal) to replace FL1501 and a CFW455IT (5KHz nominal) to replace FL1341. These filters are pin-for-pin drop-in replacements.
A. Remove top cover and unplug speaker cable.
B. Set cover aside.
C. Remove all mini coax and ribbon cables from connection points, being carefull with the small ribbon cable attached to the dsp unit if you have the dsp option installed.
D. Remove all screws securing board in the chassis.
E. Observe filter FL1501 and FL1341 locations, these are black rectangular units near the optional cw filter position. They both have 4 pins inline, with one pin offset, 5 per filter to be unsoldered.
F. Desolder all pins and remove filters.
G. Ensure that plated-thru holes are clear.
H. Install filters and solder into place, making sure that appropriate bandwidth* is selected for each position.
I. Observe solder points with a magnifying glass to ensure no unwanted solder jumpers or cold solder joints were created.
J. Re-assemble the board into the chassis and connect all coax and ribbon cables and screws.
K. Connect power and antenna to the rig, and replace the top cover including the speaker jack and power the unit on. What we are looking for before we screw the cover back on is proper operation. If the rig isn't operating correctly, remove all screws and cables from the board and double check your soldering. Correct any soldering mistakes and attempt powering on again.
L. If the rig is up and running now, replace the cover screws and enjoy your newfound AM bandwidth. This mod made a huge difference in SWBC and BCB listening, even on S9+40 sigs next to lesser powered sigs. No more 5KHz hets. You can use the wide AM filter (FL1501) by itself by holding down the FIL menu button in AM mode, a W will be displayed. This filtering scheme bypasses FL1341. Otherwise they are both inline during AM mode use, both adding to the ultimate selectivity. You still have a very narrow filter available in the standard MuRata CFJ455K5 2.4kc ssb filter, selected by pressing the FIL button whilst in AM mode.
*I would go no lower than 6KHz in the FL1501 slot, although many users may prefer a narrower standard AM mode FL1341 filter than my choice of the 5KHz unit. All 455KHz IF signals pass through the FL1501 filter, so FL1501 needs to be at least as wide as the widest filter bandwidth desired. MuRata CFW series Filters are available from many vendors, to include Icom, Kenwood, and Yaesu. One must typicaly know oem part numbers to order from these vendors though. Kiwa and other businesses who cater to the SWL may also have a variety of CFW and CFWS series filters available. I had mine on hand, pulled from parts rigs.