I have actually compared the RP91 and the DV-5700 back to back. The RP91 is a very good player, but the PQ on the 5700 is better in every respect, sharper, just as good color fidelity, and the de-interlacing much better. Most of the 5700 owners on this forum that use it as a progressive scan player have found the PQ to be among the best available for under $3-4000. It does have some issues such as a poor remote, it does not scale the same way the RP91 does if your TV locks into full mode with 480p, and the layer changes are slow on some discs. It does a very good job with DVD-A, works very well as a CD transport, and has very good sound quality with DVDs. I personally recommend this player very highly for those who value sound and picture quality. If you do a search on this forum for "Kenwood DV-5700," you will find a significant number of threads to read through.
There are reviews on audiorevolution, Stereophile Guide to Home Theater, and in the new magazine DVD ETC. coming out at the end of May. Make sure if you audition this unit that the edge enhancement is turned off, and the enhancer turned down to 0, otherwise the picture looks artificially enhanced