Here is a write-up I found that explains the changes made in the L-505ux model
LUXMAN L-505u versus L-505uX
Tonally both amplifiers were voiced the same by Hashimoto-san at Luxman, Yokohama.
Sonically: The L-505uX is quieter, more liquid mid-range and a more three dimensional soundstage is thrown. The new model is a little more refined, a little more detailed and even nicer to use than before.
Features: Both amps have most of the same features:
The L-505uX has a LECUA* discrete volume control based on the C-1000f preamplifier design no moving parts, no tracking issues at low volume. Much more transparent sounding and will sound the same at every level. This is the most important of all the upgrades and the most expensive to implement, as the individual resistors need to be measured by hand.
The L-505uX has MONO, SUBSONIC (Rumble) & LOUDNESS Filter switch on the remote control, Loudness is a wonderful feature for late night, quiet listening. It also has a few buttons that support the D-05/06/08 CD SACD players basic functions.
The L-505uX has a newly redesigned phono stage; which is both more refined, resolute & musical
The L-505uX has ODNF** 3.0 performs three passes of the audio signal, checking for noise & distortion, while the ODNF 2.1 in the L-505u does two passes.
There are only a few differences between the two amps.
--The L-505uX now comes with a metal remote control.
--Both the volume control & the selector know have a more tactile feel to them
--The L-505uX now has higher quality speaker terminals, spread out horizontally to allow easy access for large cable.
--The L-505uX no longer has the Line Phase Sensor it does not comply with CE / UL regulations..
--The L-505uX now has LINE STRAIGHT (engage / disengage the tone controls) & SEPARATE available on the remote control. There is no excuse for not using the tone controls, especially when they can be engaged / disengaged from the comfort of the listening chair.
LECUA* = Luxman Electronic Control Unit Amplification
ODNF** = Only Distortion Negative Feedback