There are just so many good preamps that work well with acoustic music, its almost impossible to recommend just one. Generally speaking, you would want something clean and open sounding, especially in the high end. You also need a lot of gain as most acoustic instruments will be softer. This is especially true if you are using low output ribbon mics on quiet sources. You mention Grace which makes very nice clean sounding preams, but there are SO MANY others too, like Millennia, True Systems, John Hardy and on and on. My personal favorite mic pre for acoustic music is the very open, quiet, and high gain vintage TAB made Telefunken V376, but those are somewhat rare (the later V676a and the Neumann made V476 are also great). But again, so many choices!
The standard answer would be something solid state and probably transformerless, but there are no rules here as the TOTAL design of the pre is what counts (the vintage Teleunken I mention is solid state unit with a transformer). The reality is there are muddy sounding solid state transformerless units and there are very clean and open sounding tube units with big transformers in them. Also when you ask open ended questions like this, a lot of people will try to sell you something.
Try as many as you can. Be sure you have mics up to the task! Good luck.
J. Mike Perkins
MySpace.com - J. Mike Perkins - HOUSTON, Texas - Pop / Powerpop / Rock - www.myspace.com/jmikeperkins