Wow! Hard to believe how Good this sounds!

Easy. You were listening to FM radio on a boom box. Anything would sound better than that.

Maybe the tweak fairies sneaked in while you were away and swapped some fuses over. 

Always keep a log. Keep a record of any changes you make to the room or system, what type of objects you move in and out of the room, e.g., musical instruments, books, unused audio equipment, new tweaks, windows open or closed, acquiring new CDs or getting rid of them, the weather, time of day, thungs if that nature. Things you might not expect can have a big impact on the sound.

artemus_5, I've had both extremes of this experience, and geoffkait, i think I should start keeping a log to make sure I am not inducing variability.

I've been away many times for 10-12 days and get excited to return to listen to some well reproduced tunes, sat down and been both underwhelmed, finding it takes a few days to get my ears back in the game maybe, and other times been blown away, like I forgot what I was missing.  Maybe it is weather, maybe it is the tweak fairies, maybe it is moving things in and out of the room, maybe there's some quantum mechanics at work, maybe my aural mind sometimes benefits from a break and other times suffers due to the trip, who knows?  It's like returning home after a long absence to a tempermental kid/pet/parent/partner/roommate, wondering if you're going to be well received or punished a bit for your absence.  

 Beer makes my car stereo sound so much better.....