Ok, first of all we need to know exactly what variables are involved. You've stated that you're moving from your bedroom to the garage, and haven't indicated any change in amp, effects, picks, or anything else that could be involved, so I'll assume that the location change is the only variable.
Have you had your bass set up (in terms of action, intonation, and neck relief) for the different tuning you're playing in? Or have you just tuned the bass down and left it at that? If you haven't had it set up correctly, my point (coming soon) is more likely to be true.
When you move locations, do you notice a significant change in temperature? Changes in temperature/humidity can cause the neck to shift (don't worry, it's normal, innocuous, and can't be compensated for). I used to find that when I went from my bedroom to my mate's garage for practice, my bass' neck would back-bow, effectively lowering the strings and thereby increasing fret buzz/string noise. Those aspects of your tone are always there to varying degrees, and have the effect of increasing how much higher-frequency tonality there is.
This is a fairly farfetched theory, but it' the first one that pops into my mind. But either way, I'd imagine it's something going with your bass. Either that, or the acoustic properties of the two spaces are just THAT different. Does your garage have a concrete floor? What about an exposed steel door?