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There was a recent (now deleted) thread discussing what the Presence Control does and how it may enhance the sound. In an effort to add something productive to the discussion and to document how the various tone controls operate, I took my HD500 into the lab and measured the Frequency Response of various amplifiers. You can find the full report here (3M big, too big to attach here). A couple of points to note are mentioned in the front page of the summary as follows: Test setup: APx525 -> Guitar Input -> HD500 -> SPDIF Output -> APx525 Input level: -52dBV. This is very small but was required to keep the high gain amps in their linear region. SNR was still better than 60dB. HD500 settings: Inputs: Guitar/Variax Guitar: normal Channel Volume = 100% E.R. = 0% Drive level adjusted to give the best THD+N (usually below 0.2%) for the cleanest looking graphs No cabs (and therefore no mics, no Cab DEP parameters) Amp DEP settings: Master Volume = 100% Hum = 0% All other controls (Sag, Bias, Bias X): default values (50%) Tone controls were set to give the flattest reasonable response. As one control was varied, the others were kept at this â€œflatâ€ setting. This doesn't represent a setting that necessarily "sounds good" but it provides a good starting point for comparisons. Also, the low level input allows the underlying frequency response of the amps to be seen. For larger signals, like guitar level signals, distortion terms will become more noticeable and add more high frequency content, perhaps making the amp sound different from what you may expect by looking at these graphs. So, use these just as a guide. Or use them to figure out what the controls do. Some are cuts, some are boosts, some are both. Some operate over a much wider frequency range than you'd expect. And now you can see what the FAC control does! Finally, I did not measure the effects of how these controls may effect the dynamic response of the amp. Adjusting them might effect the "feel" of the amp. I do not know. These measurements will not tell us. Enjoy, I hope you find them useful. ----------------------------------------------- Edited Jan18, 2018 to update Dropbox link.