Well, you could go for a Yamaha R-S700 or A-S700 (used) or A-S701/A-S801 (new). They all have the same power amplifier which would fit your speakers. I personally like the A-S1200 for it's build quality, but you probably need luck to get one used for your budget.
Not sure how much of a tinkerer you are. If i where you, i would go a different route:
- Get a UMIK-1 measurement microphone. Used to measure the sound of your speakers in your room at your listening position.
- Get a MiniDSP Flex, they should be available gain soon. This is used as DSP (to fix the flaws found with the UMIK-1), DAC and preamp to control volume. You can also use it for bluetooth and digital sources like CD, XBox, TV or PC.
- Get a NP252MP based power amplifier (check the link in my signature, best bet would probably be to import from France from Audiophonics for you). You can bridge these amps to get double the power, just buy a second one if you think one is not enough. In case you get new speakers down the road that need insane amounts of power.
With that you should be able to get a lot more enjoyment out of your DM603s. DSP is where the magic is, my man.
This is the cost you are looking at:
650€ for the MiniDSP Flex and UMIK.
500€ for a Audiophonics NP252MP.
One last thing: check your woofer sourrounds, i got an offer for a pair of DM602 S2 where the bass was totally off because the sourrounds turned into hard plastic. Probably caused by lots of direct sunlight. If that is the case for you, you best get a new pair of speakers too.