As an experiment, I tried the LF1 in this room with the twins, and it was absolutely horrible. It simply can not compete with the 500's.
Yes one answer that makes sense is that the LF1 cannot keep up with the 500's. But, maybe there were issues with cancellation and addition since you tried an odd number of subs together. Though, Randy has 3 subs and is quite happy with the results, it seems like figuring out the correct placement would be difficult. The only way to be sure is to replace one 500 with the LF1 and see if the bass is still horrible.
You probably are right that the LF1 cannot keep up with 500, if you try to integrate them though and I'm leaning towards just saving up to upgrade for two midrange subs like you suggested, but what I mentioned about is one possibility that came to me when reading your post.
