I replaced my LX5500 with a 32DVD RF Avant and would recommend the upgrade. Two things to bear in mind though - one is that the 32" wide screen has a shorter optimum viewing distance than your 28" MX ... and conversely, that the Avant has a huge presence - even when switched off. It will not suit a room with low ceiling height in my opinion.
Picture and sound quality are outstanding from any source and you will certainly notice a big difference from an MX. Mine is connected to a Sky box via an STB-C which of course also means control of the box from link rooms. Highly recommended.
Check the dealers in addition to resellers for trade-ins. Prices seem to have fallen off quite dramatically recently.
David