Tubes are individual enough to age differently so it's hard to keep a
"matched pair" (quad, whatever) of tubes "matched" after after
20-30 hours of use which argues for individual bias controls - this
also allows you to use tubes that aren't dynamically matched while
maintaining the DC balance that the OT likes. While I've had good
luck locating the bias controls at "remote" locations I generally like
to put them right beside the tube that they effect so that the person
doing the bias adjustment doesn't accidentally lower the bias on the
wrong tube and wind up with a "cherried" plate. If you can find 'em
I like 10 turn pots.
 
Rob