I have modded both Playstation 1 models: Model SCPH-1001 and Model SCPH-5501. You can read more about the output stage mods here:
http://www.dogbreath.de/PS1/index.html
For both models, the modifications to the output stage and the power supply yield very good results. In my experience, Model SCPH-5501 plays Redbook CDs and CD-R discs signficantly better than Model SCPH-1001. The modifications to Model SCPH-5501include installing Vampire CM1F RCA chassis connectors along with a modified output stage and upgraded capacitors in the power supply. These mods work well together to improve the sound. Once modded, Model SCPH-5501 sounds equal or slightly better than a modded Model SCPH-1001. Compared to a stock unit, the modded unit provides:
- Deeper, extended and more detailed bass.
- Extended high frequencies. The sound is no longer rolled off at both frequency extremes as some have claimed with the stock Playstation. There is greater air in the presentation and the recording venue acoustics are better conveyed with improved detail and extension of high frequencies.
- Higher level of detail while retaining the midrange warmth of the stock unit (this is really evident once the output stage capacitors have had some time to burn-in).
- Larger, more expansive soundstage in all three dimensions.
- Improved imaging. I believe that with greater detail, the space around the performers is better defined.
The best way I can describe the result of the mods is that it addresses the weaknesses of the stock unit and retains all the sonic virtues of the stock unit.