The unit I would suggest is not an emulator. It uses a pair of 12ax7 tubes.
And yes, tubes are hard to emulate because existing DSP modelers, at least most of them, appear to not consider the dynamic saturation behavoir of a real tube, and instead model a static saturation condition. The only DSP modeller I have come across that models tubes accurately is a couple guitar processing units from Digitech: specifically the GNX3000 and the GSP1101.
-Chris