“Welcome to...

whatever this is,” said Don Henley , making his first remarks to the full house gathered for the first night of the Eagles ’ residency at Las Vegas’ Sphere Friday night. For however much preparation had assuredly gone into putting up a 20-night engagement in the massive dome, Henley sounded in those five words just as nonplussed as any veteran of the low-glitz 1970s probably ought to be. “We’re just the house band,” he added, offering some vaguely neutral comments about “21 st century entertainment” and quipping, “I hope you brought some Dramamine.

” Not that Henley really meant to bite the hand that is feeding him. (Nor would he, likely — the Eagles’ manager, Irving Azoff, is as deeply involved in Sphere as anyone.) And there was no doubt Henley was speaking in approving terms when he made a few comments about the sound, noting that there “are 164,000 speakers behind these walls, so it should sound pretty much the same to everyone in the building.

.. You can hear all our mistakes.

” Artists can feel however they’re inclined to feel about being complemented by — or competing with — a screen literally as big as all outdoors. But when it comes to being heard through the world’s most pristine sound system, there is not a musician in the world that is going to even pretend to be skeptical about that. The Eagles’ longish run at Sphere represents a test, of sorts, for bands coming into the venue.

Not nearly as dramatic a test as t.