Going from 10 year-old memory of a system that’s seen a lot of change over the years, but I’m certain my friend Tom used and loved Koetsu RSP with Harbeth Super HL5 (original version?). He has a great ear and wouldn't like a system that was overly warm or unbalanced in any significant way. I heard those Harbeth, though I forget if he had the RSP yet or not, and they were very nice and musical, with excellent refinement and articulation at moderate listening volumes. Though, I’m not sure if they could rock out (didn’t try). I use Tannoy & Koetsu myself; very good for rocking out at loud volumes.
I know my friend missed the RSP after he moved on. I actually bought it off him. It’s an early 2000s vintage, supposedly one of the last that had any involvement from Sugano Sr. It sounds great, but I’ve also had a few modern stone platinum models, and like those slightly better - biggest difference is that stone bodies have more powerful bass. The modern platinum models are quite close to neutral, with good extension and speed, very satisfying bass, certainly not syrupy or slow, while retaining the trademark Koetsu midrange magic.
Anyways, I wouldn’t be as concerned with speaker/cartridge pairings as with cartridge/arm pairing, particularly in the case of Koetsu. They tend to like higher mass and fixed rigid bearings. E.g. avoid "pure" unipivots, avoid carbon fiber wands, etc. The VPI unipivot with dual-pivot add-on, and the heavily damped/stabilized Graham Phantom unipivot are both quite serviceable for Koetsu, though you can always do better with a used Fidelity Research FR64fx or 64S for not too much money.
I know my friend missed the RSP after he moved on. I actually bought it off him. It’s an early 2000s vintage, supposedly one of the last that had any involvement from Sugano Sr. It sounds great, but I’ve also had a few modern stone platinum models, and like those slightly better - biggest difference is that stone bodies have more powerful bass. The modern platinum models are quite close to neutral, with good extension and speed, very satisfying bass, certainly not syrupy or slow, while retaining the trademark Koetsu midrange magic.
Anyways, I wouldn’t be as concerned with speaker/cartridge pairings as with cartridge/arm pairing, particularly in the case of Koetsu. They tend to like higher mass and fixed rigid bearings. E.g. avoid "pure" unipivots, avoid carbon fiber wands, etc. The VPI unipivot with dual-pivot add-on, and the heavily damped/stabilized Graham Phantom unipivot are both quite serviceable for Koetsu, though you can always do better with a used Fidelity Research FR64fx or 64S for not too much money.