Def Leppard, “Diamond Star Halos.”

When Def Leppard got progressively softer and more radio-friendly after their first two records, there was a definite tradeoff that kept the scales balanced for another decade or so.

It didn’t thrill the metalheads, but it made good sense, career-wise. The music didn’t really suffer, because they still had enough respect for it to wrap the hooky choruses with decent riffs and changes to satisfy an incredibly wide audience.

Being one of the freshest and youngest members of the New Wave of British Heavy Metal, Def Leppard quickly broke from being the sassy little brothers of Judas Priest, Iron Maiden and Motörhead, with eye firmly on America’s left coast.

That’s where young guitar rock was mixing with vocal hooks to the point where it wasn’t just posing boys air jamming to their music. The girls were singing along as well. The music sustained throughout the ’80s on singalong choruses.

The evolution then went full stop, and 30 years later, the bad still doesn’t know which way to grow.

Going roots rock or country pop might not be the worst idea.

Yes, new album Diamond Star Haloes points again toward paying tribute to all wonderful glam of the ’70s (David Bowie, T. Rex, Sweet, Mott the Hoople, etc.) by which Def Leppard says its influenced.

But the band already did that in 2006 with its covers album, Yeah. The difference is the song selection this time and, while it’s certainly not a terrible album, it’s also not enough to even pleasantly surprise old fans.

Unless you count the couple songs on which Alison Krauss guest stars. More on that in a moment.

Opener “Take What You Want,” brigs a big rock riff; something at which Def Leppard once excelled. It’s uncomplicated, and singer Joe Elliott gets some clean space to stretch his voice around the music. It sounds like prime Def Leppard and is a hopeful start.

Same for second song “Kick,” which really shows off the fun ’70s glam influence. It’s an example of well-constructed guitar pop. The band also delivers “Fire it Up” in the same vein. Yes, a chorus a talented 12-year-old could write, but there’s a groove and fine guitar playing. The breakdown is predictable and sounds like it was written for the training montage of a sports movie.

So it’s an OK Def Leppard record so far …  no surprises, nothing brilliant, just something for fans to chew while waiting for the big summer tour to start.

Then the weirdest thing happens: Def Leppard collaborated on dramatic ballad “This Guitar” with Krauss, a roots-country star so good Robert Plant keeps making records with her instead of reforming Led Zeppelin.

And it’s really good. That makes one think Def Leppard might have a new genre ahead of them. Krauss’ presence forces the band to strip away the usual over-production of Elliot’s voice, while (finally) giving some space to guitarists Phil Collen and Vivian Campbell to get to work.

Then comes a harmless attempt at pop with “SOS Emergency.” The song sounds like something a Canadian band would’ve written to get on MTV in the ’80s.

“Liquid Dust” made me long for 1987’s “Animal.” Back then, if Def Leppard tucked their hard rock tails between their legs, they’d at least give us an exceptional hook doing so. Same for “U Rock Me.” It’s a decent hook but except for some semi-interesting guitar playing, not much to remember, unless you’re fascinated by terrible lyrics.

“Goodbye for Good This Time” is a drippy syrupy ballad mixing strings with some nice acoustic guitar playing. Both Collen and Campbell are such talented guitarists that it’s still one of the great mysteries of post-’80s rock why they don’t turn them loose more often. It’s a nice song, even if it’s a by-the-book ballad. “All We Need” is more ultra-light filler and “Open Your Eyes is a bit of a grinder at the begining, but falls off pretty softly once the vocal kicks in. It’s not much of a keeper, and neither is “Gimme a Kiss,” which offers the same old “whoa-whoas,” same old vocal effects and mid-temp structure.

Going full ballad works on “Angels (Can’t Help You Know), which would’ve been a hit 1988, but we’ve heard this too many times before.

But here comes Krauss again in “Lifeless.” She just brightens the song, and it doesn’t matter if its closer to country pop than hard rock – though it allows some airiness for strong, melodic guitar playing. It’s a good song, and worthy of a crossover single.

After an unremarkable “Unbreakable,” the fellas finish up their 15-song, mixed effort with “From Here to Eternity,” which has a bit of early ’90s “comeback Aerosmith” to it – well written and solidly delivered, but ultimately not very inspiring.

It might be time to try something else, and partnering with Krauss is a good start.

Follow music critic Tony Hicks at Twitter.com/TonyBaloney1967.