"I currently have a Koetsu Rosewood Signature mounted on an SME-5 arm. I'd like to go up from there in sound quality without having to rob a bank."
From the level of an Koetsu Rosewood Signature I think it's hard to find anything substantial better...but you have a lot of choices which sound different....people who tend to buy 5000$+ carts, in the first place to have the most expensive toy, might say something different.
I have listened to different Koetsus, Kiseki, SPU's, Ikeda, Audio Note, Shelter, Benz, EMT, Transfiguration...
Imho from a certain level of performance it's more a question of taste and not of absolute quality...not to forget system and component matching.
If you want something good from the past...I would recommend to watch out for Kiseki's and I can also reccomend there step up Transformers. I use a Blue Goldspot and the matching MCT-2 step up.