Trust me when I say this, there are far more well-informed members of this forum than myself, but I still consider myself better informed than many of the general public. Albeit, I'm relatively new to the Outlaw community, but my experience with their equipment (let alone gear from other manufacturers) has been flawless.
Audio gear, with an exception for tube-formats (please correct me if I'm wrong), really doesn't have an expiration date like the milk in your refridgerator. I would guess that the flatter sound is a combination of a few factors:
1.) Speakers - how old are they? Voice coils, motor structures, corrosion on terminals; they can all have an effect on sound quality. What about placement? You mentioned you moved. Maybe the room dynamics are different than the old HT room.
2.) Media encoding - some DVD's or CD's are simply formatted at a lower/higher sound level than others. At least that's how I have perceived it in the past.
3.) Old age - let's all hope this isn't the case. But being a medical clinician, this is one area of expertise I know I can confidently comment on. Hearing loss is generally incremental. As we slowly turn the volume up higher to miss the detail we can no longer hear, the added SPL levels place additional pressure on membranes, causing further damage. It then becomes a vicious cycle as we keep turning the volume up and, consequntly, keep committing damage to our precious ears. Let's hope this isn't the case for you. Tinnitus anyone?
4.) Upgrade hyperactivity - I would bet this is the main cause. Just like the sports car we fell in love with and bought 5 years ago, it somehow doesn't have the same "oomph" as my best friend's new sports car I recently test drove. What's wrong with mine? The answer? Nothing. As long as I've maintained my car properly, any degredation in performance is going to be negligible and purely subjective. We "think" its getting slower whereas in reality it hasn't changed. We've simply grown bored of it. Thus, its time for new equipment!
I don't promise to have all the answers, but coming from someone who frequently thinks that all systems sound better than mine (even HTIB's - yikes!), I'm never satisfied. Sound quality I would gather is a mostly subjective determination. Use the equipment that makes you happiest. If it doesn't sound good to you, then look into changing it. Best ofluck to you!