Just so everyone knows i'm not COMPLETELY full of crap, here's a quick test you can do. On a laptop, open iTunes. Start some tunes. Click View-Show equalizer. Change to flat. Turn EQ off.....notice how everything greys out...even the digital pre-amp, as if it's not applied, yes?
Now, tell me straight, Is there one iota of difference between what you hear after turning OFF the EQ and preamp? .....strange...
Moving on...
Test #2.
Same on an iOS device. Flat>OFF, no change. figures...So, change to treble booster. Wait, where'd the bass go?, now hit treble reducer.....more bass than flat...hmmm.
Test #3
For this one, have some decent quality headphones/speakers on hand, or multiple, if you want more reference. On a pc/mac with itunes, open the equalizer again. before you start playback, click Bass Booster. slide the preamp down to nil. Save the EQ/Make Preset... call it... bassnoamp, idk. Now choose Bass Reducer. Repeat process. no preamp,...save as bassreducednoamp. do the same with flat> flatnoamp.
Now, reduce the volume of the computer's line out to 0 & Increase volume in Itunes to 100%. choose an Apple preset, say Bass Reducer. hit play. increase volume on line out of device to comfortable level. switch to another Apple Preset, Bass Booster or Flat. switch a few times back and forth. get a good feel for both the soundstage produced and tonal accuracy and depth, and in particular, pay attention to the amount of relative audible change in TONE....NOT overall volume, but frequency range and so forth.
Next, switch to one of the made presets. flatnoamp...output will be much lower. Increase volume on pc line out to roughly the same level as when using the apple preset. Now, start switchin to bassnoamp & bassreducednoamp. The difference in the sound signatures and overall clarity/balance on ANY set of speakers is kinda staggering. More importantly still, is the AMOUNT of change that occurs when switching between these EQ's. Much more Immediately noticeable, with more frequencies, and across a wider range, than switching between the apple presets with a preamp engaged and lower device volume output. Also, notice how the frequencies in the higher register don't become anymore noticeable between any of the non amp'd EQs, as none of them had any modification. NOT SO when switching between Apple's presets. Bass Booster quite noticeable changes the response of the higher frequencies when the preamp is on.
Soooooo...software. It's a bitch.