Maybe a simple bath of water could enhance somehow the contrast, but the effect lasts just a short time. Another way, although I don’t know if this works for hardwoods, could be the use of paraffin oil. Lars-Ake mentioned once this method on his site. I used it recently on coniferous and results are fine.
If you say that your problem is that the rings are obscure, maybe this could not be the perfect solution, since paraffin oil tends to make darker the wood. Anyway it make it darker, but overall it enhance the contrast between seasonal rings, so you can try it. Furthermore, if you put the oil on the sample with a brush (several times, until the color of sample is homogeneous) the oil doesn’t penetrate too deep in the sample, so you can sand it out if you are not satisfied with the result. You can found easily paraffin oil in pharmacies.
Hope this helps. Best regards,