How much do I have to spend?

Components, especially amps and speakers should be matched by how well they perform together and not by price tags.  Your Tannoys are 8 Ohm, 90 db speakers so they won't require a ton of power to drive them - just good clean power with high current.  If you tell us what Rotel power amp you have, you may find you have all you need right now.

Also, a lot of what you are seeking - depth, imaging, soundstage, etc. comes from the preamp and your Audio Illusions is a pretty good one but its starting to get long in the tooth.  I would make sure its still operating at factory spec.

The Rotel is giving you 200 wpc into 8 ohms, more than enough for the Tannoys.  You most likely have power to spare on tap that your never using.  I really don't think you need a different amp.

Again, an amps primary job is to drive a pair of speakers.  Everything else, soundstage, imagine, refinement, detail, etc. comes from the preamp and you have a pretty good one. 

So if you want to improve the overall quality of your sound, I would be looking at speaker placement  and your room acoustics first.