7506 Professional owner here.
I went to my visit my aunt's house (I think it was a year ago or two). She had one of those 1980s or 1990s-looking Sony racks... I think it was a separated pre amplifier and a power amplifier (not sure if they're high-end sony's though). I wanted to fiddle with her rack because I wanted to hear how they sounded like connected there with the 1/4 inch connectors. I connected my 7506 to the phone out located in front of the power amplifier, then made sure all the artificial effects are off and treble/based set at zero (she had them all dialed to the max).
I'm not sure if I'm the only one with this conclusion... but this thing sounds like another headphone with (almost) no vices when paired/connected to Sony racks. I also use 7506s on portables (but not much after the following revelation(s))... and I think the vices on these headphones only show up on portable sources and laptop/desktop PCs.
Here's the impressions that I consider "improvements" I noticed after testing with her Sony racks... (in no following order)
- The "low end" got "tighter", "thicker" (more like "full-bodied" but not that thick) and even extended "deeper" to boot!
- The "highs" felt that they were tamed.
- The body of the music (I played all genres... high quality stuff, ranging from Bread Greatest Hits, DCC-issued Metallicas, Andy Gibb Greatest Hits, and some R&B, old school hip hop and some selected Southern hip-hop/rap/crunk)... felt "smooth" overall ! I think this 7506 still got some minuscule "rough edges" somrwhere... but Like I said it was "smooth" overall. Most of what's considered vices with 7506 sound has been greatly minimized, or even diminished.
- For once I was able to "distinct" the notes and pinpoint where they came from.
- So in short... Low = "tighter"/"thicker"/"deeper". Mid = "there's actually mids!", not that prominent... but more like present. High = "tamed", "smoothed" but there's still some small "sharp edges" present.
Because of this I retired my 7506 for portable duties and is hidden in my closet for now. For recreational purposes I think the 7506s strong points doesn't do it justice when not plugged in to racks like I described above.
I tried to buy my aunt's 20+ yr old Sony rack system to compliment my 7506... but still no luck. She just wouldn't give them up.
I tried to to look for a rack like my aunt's for sale online (and for cheap) and still no luck here either.