There is a Marantz M1 in the classifieds at a discounted rate. Could be a good bet.
Not to make you change your decision though - as you can't go wrong upgrading in any sequence.
- If you are looking for a different sound. A speaker might be a better bet to invest in. Your amp though will change the character of the sound, but it won't really be a huge jump. A good pair of bookshelves with the rest of the gear will make a more substantial change in sound I believe.
Also personally I am a big fan of DSP/room correction - it allows you to tune in your sound to perfection. You can't change your room, but you can use the tech to suit your gear to your room. You can have a new sound every day if you like. So in your future plans - do keep some budget for DSP. If nothing else you will learn how sound works in your room