I guess it boils down to personal experience and taste to a certain extent.
I like to start with well maintained amps that have been kept well or restored to full potential in case of vintage ones. I choose cabinets and speakers either old or new based on what works and has worked for the past twenty five years for me and other I have worked with/for. For the vintage ones I pick old Mullards or Brimars, for the modern ones it depends but right now it seems Winged Svetlanas and Tung Sols are acceptable sounding. Trying various combination with different amps and cabinets to see (or rather hear) what works in a given production context is key. Sometimes an old vintage seventies cab with Celestion will do wonders, other times a newer EV equipped modern cab will be just the ticket. Personal favourite speakers of mine are Celestions, especially but not restricted to Heritage 55, old pre Rolas, Fanes, Tayden and Ev. I really like Eminence for Bass.
The same goes for microphones, even though I have favourites I will always try most if not all combinations having access to vintage Neumanns from 1938 to present days as well as other industry standards. Having said there are times when an inexpensive microphone with a given amp and cab combination works better than an expensive one so it is always important to keep an open mind and aim for the final result rather than looks or "perceived values". Sometimes I use multiple microphones, other times I go for a simple setup if I feel it works better. Same applies to preamps although some seem to me to work most of the times so I will grab those ones first, namely a V72,
Telefunkens, either a 676 or a 376, an Helios if I am going for that '67 sound, a Neve '73 or '84 or similar designs like the Heritage, Daking or a very cleaner one like a Millennia or Red or the Tore Seem in our desk. To me a V72 sounds very different from a Millennia. If I feel it is worth it I will do a pass through analogue tape.
Placement varies and again depends on the given context. Monitoring is in a separate (control) room checking usually on nearfield as well as mains (PMC, ATC) and some reality check on smaller mono speakers.
Musicians vary from crappy ones (our engineers 
) to gifted friend, visiting musicians (some long term customers always willing to help) to some session players we call in for customers in need. Guitars and basses also vary starting with common ones like Teles, Strats and Les Pauls with some less common new and old pieces to see how a profile will react to very different instruments.
As mentioned above experience helps, having worked for years in tracking and mixing and having had the privilege of assisting, making tea and sandwich for great producers and engineers has been invaluable, especially many years ago when I started and apprenticeships were easier to find than today.
do you use/play all of them?
