Wow, a lot of postings here that agree with my own personal taste/experience. I like the Koetsu stone series because they are reasonably "fast" (dynamic and clear and not overly warm). But, the other Koetsu cartridges sound very good too. As Lewm points out, the Urushi is a particularly good overall performer.
Outside of the Koetsu line, I think the Transfiguration Phoenix should be considered as a candidate for your upgrade. This is a lively sounding cartridge with a slightly prominent midrange that gives it clarity without being excessively bright. I like the Lyra line too, but, these are on the more detailed and analytical side (more open top end, but can be bright or slightly strident). I own, and like, the Lyra Titan.
Outside of the Koetsu line, I think the Transfiguration Phoenix should be considered as a candidate for your upgrade. This is a lively sounding cartridge with a slightly prominent midrange that gives it clarity without being excessively bright. I like the Lyra line too, but, these are on the more detailed and analytical side (more open top end, but can be bright or slightly strident). I own, and like, the Lyra Titan.