Hi Hifibear,
I have some experience with Vandersteens, although not any of their Ones. Still, I have a few suggestions.
First I would contact Vandersteen and ask them for minimum recommended amp size based on your room, music, and typical listening levels. Mention your intention to utilize a subwoofer since that can lower the power need from your primary amp. Keep in mind you can usually get by with a lower rated tube amp than SS.
Second, your budget of "as low as I can go to get good sound" is not terribly helpful. Possibly the best you could do for the least money would be to keep your 3020 as a preamp and add an amp such as a Dyna ST-70. That might need new tubes and caps but should cost less than $500. If you are lucky you might find one someone already updated and save on that.
For good sound with Vandy speakers there are some traditional recommendations - Audio Research, c-j, MusicReference, and Quicksilver. Smaller versions of these amps will still cost more than a ST-70 but should be more rewarding.
I'm focusing on tube amps since you mentioned wanting spacious sound.