Almost as an aside, Steve Deckert of Decware advocates introducing an extra gain stage control inserted after the source, in addition to the one at your amp (ZBOX). This allows you to do some "gain riding" as he calls it, letting you either turn up the gain going out of your source while also turning the gain down going into your amp....or vice versa. I haven’t tried it that way (yet), but he says that increasing the source output gain while lowering the amp input gain, definitely changes the overall character of the sound...in the right adjustment zone, he says the sound in that instance is much more dynamic. Presumably you can take this idea too far and it may begin to deteriorate the sound in some other regards the further your adjustments go, but his point is that you have a choice of what sounds best to you. If this arrangement produces that result, then perhaps it is not hard at all to accept that you are hearing, ghosthouse, a slight difference in tonal response in your set up (and that Al would be correct by way of what he has already pointed out, as usual, but I defer to Al and others on most matters ee).
Here is the ZBOX link which has a description and a link to the owner’s manual: www.decware.com/newsite/zbox.htm Steve makes mention of both impedance and frequency response.