I have been away from Head-fi for quite a while now. After several years of absence, I decided to pay back a visit to this wonderful site.
I am still using my Proline 750 as my main headphone and the sound it produces never ceases to amaze me. Although I have tried many other headphones which are highly regarded by many head-fiers, I must confess that I keep coming back to my Proline 750 with its amazingly addictive sound signature, which is simply pure, extremely accurate, body-quivering lows, and deeply immersing especially due to its S-Logic design (and, yes, S-Logic does work).
One thing I would like to emphasize when using the PL750 is that you MUST hook it up to a high-quality source. The rule of "Garbage In Garbage Out" applies exactly to the PL750 since they so accurately reproduce whatever sound is thrown to it, whether it be background noise or the person coughing behind the recording studio. As an example, my PL750 is usually hooked up to my Graham Slee Voyager headphone amp which is fed by my old trusty Nationite S:Flo2 (dual-mono design with 2 separate Wolfson WM8740 DAC chips per channel). With appropriate high-quality FLAC recordings, one can only be amazed at the music reproduction you get. And, by the way, always set your equalizer to "Flat" for proper sound reproduction or otherwise sound will be distorted.
Just wanted to add my humble but very honest opinion on the Proline 750 after having several years of extremely satisfying usage.
By the way, kudos to Peter for his continued support for the Proline 750, a definite real gem.