Some to many will disagree with my approach below.
But with what you are saying, I think perhaps the best speaker upgrade you can do for yourself is to consider purchasing a new amp first. Using your words, a super nice amp.
Face it, if you know such super nice amps exist and if you think you can at all afford one, you know you'll probably buy one someday soon anyway so why not focus on that now?
I mention that because I am in the camp of those few who the amplifier is the key to any system. Whether good or bad. I also believe in the addage:
"An excellent amplifier can make mediocre speakers sound almost excellent. But an excellent pair of speakers with a mediocre amplifier will never sound better than mediocre."
If that is true, and I believe it is, I would therefore recommned looking at the amp first, see what sonic improvements it provides your current system. Then focus on new/used speakers to match that new amp.
I believe with this approach, you may also save yourself from a few unnecessary upgrades along the way. And that hopefully translates to better sonics, a faster path to getting where you envision being with your system, and hopefully spending less money getting there.
-IMO