I really like the 300Hz optional roofing filter in my FT-DX10 but have been annoyed by the drop in gain when it is switched in. Andrea HB9FBD has suggested a solution. Now there is no change in gain when switching between them.
Fortunately it does not involve opening the rig or making any physical or permanent changes. Instead, the adjustment is done through a "hidden menu" that is used for service alignments and adjustments and easily available from the front panel. The menu is described in the FT-DX10 Technical Supplement available on the internet.
The relevant section from that Supplement is reproduced below:
To summarize the steps:
Press and Hold these three buttons while turning on radio: STEP/MCH, CS, FINE Rotate Function knob to select 2-027. (Default setting is 91). Turn MPVD ring to select new value (115 seems to work well). PUSH and HOLD Function button to exit menu.
The menu will look like this:
(In case you are wondering, the nylon tie around the RF gain control is to make its setting more visible.)
Update: Several on the FT-DX10 forum have suggested that increasing filter gain could reduce the S/N ratio or increase filter ringing. Especially if you are a DX-er or contester where you encounter lots of signals and congested bands, you might want to compare which setting best suits your purposes. It's an easy adjustment.
For what it's worth, the default gain setting for both the 300Hz and the 500Hz are the same - 91. Was there an intentional decision by the designers not to adjust for insertion loss with the 300Hz, or did they just not bother?