Any number of speakers to try, and whatever anyone suggests you still need to hear… Big ATCs SCM40 and upwards, perhaps, depending on budget, though likely less bass extension.
However you can certainly move up in PMC: Twenty or twentyfive.26 might be a step up, or definitely Fact 12, while MB2 is superb. The prices of course likewise, though if it were me I’d buy secondhand - age really isn’t an issue… The other obvious step-up from the OB is the EB1i.
Whether the Star can get the best out of any of these PMCs I have no idea.
If it were me I’d be keeping the speakers and upgrading the electronics. I wouldn’t put twenty5.23s on a Star, let alone any of the much more illustrious models mentioned above.
I’m still utilising my PMC OB1i with a SN3 + NDX2. I’ve dabbled with the idea of new speakers numerous times through various system changes but they always kept delivering what I expected of them!
If you have the space and inclination, taking a look at a SN3 + NDX2 might be an interesting option, I’m supremely happy with what I’ve got now, headroom above it of course (as always in this game) but paired with those PMC’s it does a fine job.
Recommending speakers is always somewhat open ended, I’ve heard speakers dealers and friends have raved about and fallen off their chairs in excitement over which left me unimpressed, I’d expect you’d be the same. If you’re serious about keeping the system as it is with a Star at it’s heart, and are looking at speaker options, where possible borrow what you shortlist and give them a good week or two at home and take the time to make a choice you know you’ll be able to live with.
Your ears and your listening room play a key part in what will work well in addition to aiming for appropriate systems synergy and amplification matching.