a few commercial examples as requested:
http://www.mercenary.com/vt1vactubmic.htmlhttp://www.mercenary.com/alltub40dbmo.htmlhttp://www.mercenary.com/rmstubule.htmlnot tube but a respected (and cost-effective) minimal solid state design -
http://www.fmraudio.com/rnp.htmmy picks for a minimal design - some of this might be a given but I'll spell it out anyways -
XLR in, balanced TRS out
switchable +48v phantom power
input polarity switch
input attenuation (switched -15db pad would be enough, variable is much nicer)
variable gain (6-12 stepped is nice but not necessary - notice that the Manley linked above is only 5 steps)
Hi-Z unbalanced input for use with electric guitar/bass (I'm curious how much this 'active DI' would add to the cost of the design?)
signal and clip indicators
rackmount isn't completely necessary but that would be a big step towards making such a Bottlehead product very studio friendly - a 1/2 rack form factor that is optionally rackable or desktop is, in my opinion, the approach most likely to find a home in project as well as professional studios - 2 channels of a particular preamp flavor in 1 or 2 rackspaces high is very desirable to many studio owners.
and . . . here are my votes for extra features beyond the bare bones -
variable input impedance
unbalanced insert jackVU Meter
but really it doesn't need much at all... I have a solid state mic preamp within spitting distance of me right now that besides the in/out jacks only has a gain pot, a clip LED, and a phantom power switch