Since Sirius XM has been doing this Kiss channel all month, I've had a blast (my family, not so much) listening to nothing but Kiss in the car. The band's catalog is literally in my blood at this point, but there has been the odd song that made me go 'wait, what album is that off of?'
In every case, it's been Monster and Sonic Boom. So I made it a point to relisten to those both.
My opinions haven't really changed (I remember loving SB, and feeling pretty 'meh' on Monster), but I probably only gave each disc 3 or 4 spins when they came out. I can tell you this, I've been missing the boat with Sonic Boom for years. That album sounds great. The production is spot on, and both Paul and Gene kill their vocals.
Now Monster, well...that's still a bit patchy for me. I really wish there was a more fitting Swan Song for the band, but as a one/two punch, I give Kiss a lot of credit for those two records. They were albums they certainly didn't need to record, but I'm very glad they did.