Chevy
I also remembered/realized, while reading all this good info, was that most decent studios (even modest ones) will likely have a decent tube preamp before the cpu interface.
They may do, or they may not. Some of the most highly regarded consoles and press are solid state.
Chevy
Is there a bit of magic added with a hardware tube preamp that seems to be missing and can't be simulated otherwise?
Sort of. Valves (or tubes to use the US term) have qualities that are valued, but more in the guitar amp field than mic pres. Their most liked quality is their ability to distort in a way humans seem to like. Emulating valve behaviour is something that software and hardware companies have been trying to do for years. Personally I think that emulations that don't have too much saturation going on and are based on circuits that are designed for low (by valve levels) distortion and saturation can be pretty good, but guitar amp emulations generally lose out to the real thing or even analogue solid state emulators such as the Sansamp range. Others will disagree with me :-)
If you want a little valve magic on a low budget there are the Art tube press and the Behringer equivalents. They actually don't sound to bad, but the Behringers in particular are fitted with a very cheap and pretty horrible example of a valve. Switching it to a better quality one can make quite an improvement. That's the thing about valves, just because two have 12AX7 (or ECC83 if you're European) written on them and are supposedly the same thing doesn't mean they'll sound or react to input quite the same. Selecting and comparing valves is a can of worms in its own right.
Again though, the kind of difference a valve preamp makes over a decent solid state one in a typical interface doesn't leap out at you, it's a subtle difference. And if the interface the pre is connected to isn't very good then.... And the mic also needs to be good, otherwise all you're doing is very nice amplification of a not very nice microphone... And you need good monitors and experience to understand what its doing....
If I were you I'd sort out a couple of decent compressors (the Sonnitus you already have is one), a limiter and a reverb (Breverb which you have already is good), learn to use them then start looking at where to go next..