Using DSP on low bitrate file (MP3<160kbit/s) seems to improve the quality slightly. I find OK listening to very high bitrate file or lossless without DSP.

Then, I do frequency graph test on MP3 b4 DSP and after DSP.

a 192 kbit/s MP3 b4 DSP, I can see a clear cut at 19 kHz, but after DSP it's frequency graph looks like original wave with no cutoff at all!

a 128 kbit/s MP3 after DSP, it looks like a 192 kbit/s MP3 with cutoff at 18 kHz???

Is the DSP synthesising High - end frequency like MP3PRO SBR?

Yes many DSP regenerate high freqs in low bitrate files, but it isn't an accurate representation of the original one, more like an extrapolated version.