"So, how do I know if its real or imagined? Its not a big deal, no real investment and only a very subtle perceived change, but it does relate to other issues that are often debated, sometimes heatedly, on this forum. How do you account for, or eliminate expectation bias, other than a proper double blind test which I'm guessing most of us are not going to do when auditioning equipment?"
Good questions.
I'd say you can't ever know. None of us can. Our entire existence on this planet comes to us through perception. But that's not really a flaw. In fact, since it's so universal to our experience, it's actually a strength, and in audio, your best ally.
But too many people make the mistake of thinking that perception is a short game (like with double-blind testing, for example) It's far more often a long one. That's why simply owning your gear/system for years gives you a far better overall impression of its behavior than you had the day before you bought it...or the day after.
And perception is too easily influenced by all kinds of factors that affect our mental state or mood for it to be a short-run thing. And, we easily might miss things when we're anxious, stressed or excited than when truly relaxed. You might think of that as a beta-waves-vs-alpha-waves kinda thing. But, after many cycles, over months or years of listening, when in a variety of states, we become more convinced we've built up an accurate impression for ourselves of what our gear/system means to us. And your impression may differ from mine and mine from someone else's...but it's all good when we each use our own to arrive at whatever kind of sound we might want most for ourselves. And the basis for that of course, has always been nothing more than individual preference.