An unusual problem

I actually just got rid of an awesome REL simply because it provided a 15% gain in sound quality for me and a 75% gain in irritation for my room mate.  
I’d look into finding enjoyment in a set-up that doesn’t bother your neighbors.  Sell the sub and put that $ into upgrading the transparency, resolution  and musicality of your set-up!

Magnepans might be something to consider as I’ve heard that they don’t transmit bass through walls quite like conventional box speakers.  Magnepans are more directional so they may not sound quite as good when walking around your loft but that directionality may help you out as well since it sounds like you have glass along the entire 1 side of your listening area.

Good Luck!

I actually just got rid of an awesome REL simply because it provided a 15% gain in sound quality for me and a 75% gain in irritation for my room mate.

 

So there you have it- REL: the sub so good even when its bad its because its so good.

There's next to zero chance this will work but just in case the wall is super thick concrete or whatever and the glass really is where all the sound is coming through.... 

Bass is making the window pane vibrate like the skin of a drum. You can stop that with a bead of silicone all along the wall/glass corner. But that will mostly stop the glass rattling against the wall. Which will be an improvement for sure. 

But then the window pane will still be vibrating, only now the wall will be like a pivot point. The easiest cheapest most unobtrusive thing you can do is glue a 2x2 into that corner all the way floor to ceiling. 

Try the silicone first, that will at least stop the rattle and give you a clue if anything else is even possible.

Perhaps a Townshend isolation platform for your subs would be a way to go.
I own a lot of their products and can attest to their efficacy in isolating vibrations. They also work great on isolating components- resulting in a much cleaner sound.
Bob

I hate to be the one to point this out but wouldn’t $50 of simple springs be a little bit more cost effective? Hel-loo!