Warthur On The Grand Illusion, Styx did a fine job of walking the tightrope between poppy AOR and prog-tinged hard rock, and on Pieces of Eight they try to do much the same. As with its predecessor, it's a carefully judged balanced: there's just enough hard rock touches to make it feel credible in that arena without compromising the radio-friendliness, and just enough little flourishes to give a hint of prog whilst still prioritising pop hooks over prog complexity. (That said, going terse can bring its own benefits: on I'm OK the band cram a bunch of little movements into under six minutes, yielding a song which gives the feeling of a multi-part prog epic without demanding the runtime of one.)

This sort of alchemy makes them exactly the sort of band the term AOR was coined to describe - because they're not prog rock or hard rock or pop rock or soft rock to be unambiguously described by any of those terms, but are enough like all of those things that they're clearly some flavour of rock aimed at a somewhat more mature audience than more singles-oriented genres.

At the same time, "AOR" tends to be associated with very commercially-oriented material, but that's a little unfair here. Sing For the Day isn't the sort of song you do expecting it to be a hit, and likewise if you were just trying to churn out viable radio material the 1-minute instrumental The Message is a weird thing to spend time on; in 1978 if you were wanting to chase the big money you'd be making disco or new wave. (Styx would eventually do exactly that, but they don't do it here.)

In fact, you could argue that despite being as radio-friendly as it is, Pieces of Eight is commercial despite itself - it isn't necessarily being anti-commercial, but it is being anti-bandwagon. It's carrying the torch for progressively-tinged hard rock in an era when many bands working that style were shifting away from it - Lords of the Ring puts me in mind of the more grandiose efforts of early Queen, for instance, who this same year were putting out material like Fat Bottomed Girls. On the whole, the album doesn't capture the same lightning in a bottle as The Grand Illusion, but it comes very close.