Depending on the type of tube, there are really very few things that matter. For a triode, mu is the most important, then there are things like transconductance, etc. that are secondary concerns. For a power tube, you have transconductance, load resistance, output power and maybe a few other factors. If you can match the values between submini and regular tubes, then there shouldn't be much of a difference sonically, if any. I am going to do a submini JCM800 after I get a little more settled, though I also plan to do a submini Mark IV preamp and a submini deluxe reverb preamp at some point. Tubes are tubes, they operate on the same physical principles regardless of package. It is similar (though not identical to) SMD and through hole components. Of course, the differing plate voltages and such will change, so you might want to play around so that you get plate voltages that lie at the same point on the characteristics graphs as those for a regular tube (i.e., try to find a similar response characteristic and the plate voltage that produces it as opposed to trying to straight up match voltages or simply maxing voltages). Hope that made sense.
