The Ashdown RM500 is a great amp, it’s fuller sounding than the Markbass, has better eq points - imo of course - and with both drive and compression as well it is much more versatile. Having had both amps, you could get the Ashdown to do what the Markbass does, you couldn’t get the Markbass to do what the Ashdown does. Also, due to the pre-shape feature on the RM500 it can get into ABM territory.
They're both good amps though, get a good deal turn up on either and I don’t think you’d be disappointed.