I run Tannoy Canterbury GR with Rogue Apollo Darks, which are similar to your ST100, just bigger. It’s a good combination. I like how the amps give gobs of power for rock & metal, with a neutral tone.
For preamps I’ve tried a number, and I’m still a little confused lol. I think a good match is a very slightly sweet, warm tube pre that’s - if anything - just a touch relaxed on top to work well with the Tannoy’s horn. You also want an extremely low noise floor since the Tannoys are very sensitive.
I’m pretty happy running an ARC Ref 6 right now, which does everything well with just a *hint* of sweetness and a top end that’s not too aggressive. The VAC Renaissance V had the perfect warm, beautiful tone, but couldn’t rock as well as the other preamps - a deal breaker for me. The Rogue Hera II was too noisy. The RP-9 is great but I’d like it to be just a touch more relaxed on top, and it’s still not as quiet as the Ref 6 (none of the Rogue pres are that quiet). Still evaluating that one, though! An RP-5 with Mullard tubes (even the Russian reissues are good) might be a good option. A used RP-7 should eventually turn up in your price range, which should be better. I wouldn’t step down to the RP-1 with your level of gear. IIRC the Rogue Athena had a lower noise floor (they screwed the noise floor in the Hera I to II upgrade, which I now regret) and may be a good candidate if it pops up used. Also sometimes the old VAC preamps pop up for a good deal; not all of them share that circuit of the Ren V (e.g. the Renaissance III has less of the beautiful tone but could certainly rock).
And the Cronus III may be great, but I think the ST100 will be a better amp for your Tannoys. A long time ago I was running an Atlas (Cronus without the pre section) on Kensington SE when I got to try my friend’s Stereo 90 (both non-Magnum). It was a very notable upgrade in sound!