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You are talking about single channel preamps, mono, correct?  The first thing to think about is that with a single channel signal you can have no inter-aural time or amplitude differences, so in the physiological sense there can be no (true) sense of depth or soundstage.  A correctly reproduced mono signal should be a very narrow image between stereo speakers.

So any sense of depth must be because of an alteration to the mono signal the reminds your brain in some way of the sensation of a stereo or binaural signal that could convey true interaural differences that correspond to depth or position.

My only guess is some type of modulation of the signal, but there is no obvious reason a discrete amp would have any modulation compared to an IC. Usually you would use a discrete op-amp because you needed a higher supply voltage, or wanted beefy output transistors so you could drive a lot of current.

 Transformer possibly if it was not the highest quality, but good transformers like Jensen do not typically have a lot of distortion or dynamic range compression.

So I guess the short answer is there is no obvious explanation for anything like what you describe.