For my part, I have found an improvement between 3 pre-amps I've had over 25 years, in increasing quality: an Acurus L10, a Parasound P/LD 2000, and PS Audio PCA-2. The main improvements have been in lack of distortion, which has yielded smoother, less fatiguging sound, and hence more realistic imaging and soundstaging. I've used the same recordings I know well to compare them, and the only equipment variable has been the pre-amp. Honestly, with rock/pop and heavily processed music or movies it's more difficult to hear big differences between the Parasound and PS Audio. The Acurus has been consistently brighter, with more distortion during musical climaxes or loud volumes. But for me the most revealing recordings are classical recordings of human voice, both solo and choral, solo piano, and complex orchestral recordings (Mahler). Distortion with components can really become apparent with well-recorded voices, especially opera and solo voice at full-tilt climaxes. Distortion also mares well-recorded piano. With the Acurus voices at full-tilt will distort, especially in the higher registers. Piano also sounds less natural with the Acurus. The Parasound is smoother, with much less distortion. The PS Audio is glare-free, with voices and instruments also sounding more present, realistic, and distortion-free no matter how loud. Orchestral climaxes are produced with deeper soundstages, better instrumental color, imaging, and definition. You can hear more concert-hall sound decay. But as I said, this is relatively dependent on the type of music. If I only listened to rock/pop and movies, I could be satisfied with the Parasound (which I now use for my bedroom system and as a backup for my main system)--the Acurus I've found too bright. But since I also listen to classical and opera, distortion-free and glare-free sound is paramount. It's also about diminishing returns, how much you're willing to spend for improvements that might be incremental. And about investing in equipment that will last. I've had the PS Audio for 14 yrs of trouble-free operation, and bought it refurbished from Audio Advisor at a good discount. Haven't had the upgrade urge since.
