Hi Nolitan,
I have both had the 505u and the 507u, and they sounded very similar to me. I did not have them at the same time, there was a month between the two, but they both struck me with the warm, detailed powerful musical sound. In the time between I had listened to a accuphase e408, but that was a big dissapointment in comparison to the 505u. The accuphase seemed to lack the power to really control the speakers, especially in the bass. Strange, because the e408 has 180wpc and the luxman 100?.....but luxman always understates the rated watts. I sounded more like 300 watts. Then I decided to buy the 507u directly from japan.........and I still love it.
To be short. If you really want a full bodied, musical sound with a deep and super controlled bass, buy one of these luxmans. The only big difference between the 505u and the 507u is that the build quality of the 507u is higher. More like the accuphase e408 build quality.....