Thanks for the room description. Incredible, ain't it, that at three different shops no one asked about this. Along with a speaker's intrinsic response, the speaker/room interaction is really the most sonically determinative aspect of the entire system (any system).
The window treatment is a good idea, and it could allow you to aim the speakers a little more straight ahead, which would also reduce the perceived treble, though perhaps at the cost of some imaging precision. It might help to do something to damp or break up the reflection from the wall behind your chair as well. Unfortunately though, in addition to reflecting treble, windows leak bass, which the blinds won't affect (the fireplace might do so as well). It's probably good that you're set up firing across the short dimension of the room, which I was going to suggest if you weren't. I hope the area rug lies between you and the speakers. Having a wide placement separation between Thiels is not uncommon. Your listening distance is about the max recommended, but that's appropriate to your room size. Having the speakers 3 ft. away from the front wall is pretty standard, though moving them a little closer to the wall could increase the perceived lower frequency balance a bit.
But basically it sounds like you're working with what you've got, and the necessary evil (just kidding) of getting a sub is the next step, unless you were to change speakers (and even then you might still want a sub in this situation). It's no mystery to me that you preferred the sound of the Thiels in the store. The 1.6's aren't very demanding of power, though your room is, but I'm sure they appreciated the upgrade in amplification quality. You might try getting the outrigger stands if you don't already have them, as they could help slightly with the bass by increasing the footprint stability of the cabinets and hence LF wavelaunch. Best of luck with blending in a subwoofer and soliciting more advice on a new thread (lots of folks around here know more about subs than me :-), and let us know what happens, and when you get that CDP too.
The window treatment is a good idea, and it could allow you to aim the speakers a little more straight ahead, which would also reduce the perceived treble, though perhaps at the cost of some imaging precision. It might help to do something to damp or break up the reflection from the wall behind your chair as well. Unfortunately though, in addition to reflecting treble, windows leak bass, which the blinds won't affect (the fireplace might do so as well). It's probably good that you're set up firing across the short dimension of the room, which I was going to suggest if you weren't. I hope the area rug lies between you and the speakers. Having a wide placement separation between Thiels is not uncommon. Your listening distance is about the max recommended, but that's appropriate to your room size. Having the speakers 3 ft. away from the front wall is pretty standard, though moving them a little closer to the wall could increase the perceived lower frequency balance a bit.
But basically it sounds like you're working with what you've got, and the necessary evil (just kidding) of getting a sub is the next step, unless you were to change speakers (and even then you might still want a sub in this situation). It's no mystery to me that you preferred the sound of the Thiels in the store. The 1.6's aren't very demanding of power, though your room is, but I'm sure they appreciated the upgrade in amplification quality. You might try getting the outrigger stands if you don't already have them, as they could help slightly with the bass by increasing the footprint stability of the cabinets and hence LF wavelaunch. Best of luck with blending in a subwoofer and soliciting more advice on a new thread (lots of folks around here know more about subs than me :-), and let us know what happens, and when you get that CDP too.