Pommy99- The RXL you have, it's not this http://bontrager.com/model/07136 is it? I can't imagine it being spongy; or do you have the WSD saddle?

If you have the WSD one, how about trying the mens RXL (link above). It is firm, but has some 'give' built into the base, with very little padding. The center of the length of the nose is softer for your soft parts and I find it is just the right amount for me and I absolutely love this saddle. I actually have the RXL on my Ruby, and the RL on the MTB and commuter. They make it in 3 sizes (I use the 154 for my wide bones) and I think their middle size is 143.

If the thin mens RXL is what you have and are calling spongy, you have buns of IRON!