I don't have a good idea for plan B, yet. If this transmitter can't deliver the goods, then no FM transmitter that doesn't need a license will do much better. If you felt like experimenting, then perhaps things could be improved by coming up with a better antenna for your receiving unit, but that would be a lot of work I think.
I like the idea of using a different frequency band, but my experience with cordless phones indicates that 900MHz or 2.4 GHz are both going to be only a bit better if at all. It really comes down to the receiving end in these cases too. There are many 2.4GHz audio relay units but they all use low power, not enough to guarantee joy in your application. I looked at some models of wireless headphones made by Acoutic Research and they all claim "up to" 150 feet, which isn't going to do it for you.
The only other thing that comes to mind at the moment is to use a 2.4GHz wireless audio link to get the signal into the middle of your work area, then pipe the audio from that into your FM transmitter. Since the FM transmitter is much closer to where you are working, the signal would be better. Can't help but think that is kind of kludgy though.
Here's an outfit that sells cheap 2.4GHz audio links, but be warned, their website is the worst that I have ever seen. Their stuff does, however, work OK.
**broken link removed**
You might want to explore their stuff to see if they have anything that will work. Their 2.4GHz stuff, that I have seen, is very directional (narrow beam antennas) so tends to be good mainly for point to point, not broadcast over a wide area.