That seems like a great price. I often see the 500 models going for that much. I really like my ABM 500.
The only off-putting spec on the 900 is weight, which obviously isn't an issue for you based on your cabs.
I often run through the 610hlf provided at a local venue, and while I haven't yet used my Ashdown there, everything I have experienced so far makes me think it will be an outstanding match. I briefly had a Streamliner, and that through the 610hlf was among the best tone moments I have experienced. To me, the Ashdown tone is in the same ballpark as the Streamliner (just much more flexible, a bit less compressed, and "better" for me for a variety of reasons).
Only thing that bothers me about the ABM 500 is fan noise. Mine has the loudest fan I have heard, but it is also about 12 years old. I think the newer models have quieter fans. Also, my power supply seems to be failing, but it is an old amp that has changed hands several times.
I like the tone enough with eq bypassed, but when engaged the frequency points are pretty useful (at least for my cabs), but I do have difficulty dialing-out some of the extreme lows with the BASS knob (40Hz, I think). It's hard for me to find a sweet spot with that knob, like it's easy to remove too much and the amp loses its mojo.
The valve drive is great, very useful, and the overdrive character is pleasing to me.
The octave feature isn't very useful for me. For octave effect, I prefer the performance of most pedals by a large margin.
