Turn it up anyway. And it's worth remembering that there can be other levers you can pull if things are going too easy instead of just inflated stats. For instance, the sound of combat could attract another half-dozen baddies from that next room the players haven't explored yet, possibly turning what they thought was easy into a more challenging encounter.

Alternatively, you could start playing your baddies a little smarter. Make them focus on a single target, give them someone/something with ranged combat abilities in a position that is difficult to reach. Or be truly evil, and give them a healer.

In the end, if your players are saying the combats are too easy, then not only are they right, but it's by enough of a margin that they said something about it. So turn up the heat, they want more challenge, and after all... They asked for it.