The SBM1 is a 16-bit device (outputs 16 bits), so there is no point in using a 24bit recorder if you plan on continuing to use the SBM1.
That said, I think most of the modern recorders are 24bit now. And again, most of these will have a mic attenuation circuit, but few have dual 1/4" inputs, the MT is the only one I can think of. I'd highly recommend the Sony D50 which I have and love. It's outside of your price range new, but you can get a used one for that price. Single 1/8" input and optical digital in only though.
You might want to contact Busman Audio (see the Retail Section for his posts) and see if he can mod your Nak 300's to use a different termination -- perhaps 1/8" as it is more common on recorders.