I've been playing an Ashdown EVO-II 500 for the last two years, and I recently got an SVT 4 Pro. I am not looking back yet. To me, the Ashdown always sounded too wimpy and I didn't like the EQ section. The SVT4 has way more balls and the EQ is much more versitile. The SVT4 sits in the mix much better in my rock band. I think Ashdowns sound smoother and work well with mellower music, but I prefer the SVT4 all the way for rock. I bought the Ashdown used on ebay (US) for $700, and the SVT 4 also on ebay for $750, so the price difference really wasn't a factor. Now I'm GASing for an SVT II Pro (tube + EQ = awesome).
I've been playing the same Ashdown head and Ampeg gear too. My conclusion is exactly the same.
I am stuck on cab choice because I want a smaller ampeg setup and dont like any of there cabs besides the 810.