I have been using an early non-ND FT-817 for many years - the one upgrade I have fitted is the W4RT dual SSB and 500hz CW filters which really does make a difference.
I also use a BHI external speaker/DSP unit (normally set to position 2) which plugs into the FT-817 external speaker socket.
The external DSP speaker requires a 12volt supply and does have it's own headphone socket. It can also be used with any radio. The higher the setting on the DSP unit, the greater is the "warbling" sound effect and other artefacts, but background hiss is greatly removed (especially on AM) which reduces the fatigue from prolonged listening and you also get a useful external speaker which is better than the FT-817's own internal one. When "trawling" the bands I would normally not have DSP switched on, until I find a station of interest or have a QSO.
Hope this helps.
73's Alan G0TUD