In my experience any tube sounds best when:

• They are solidly biased at dc. A good constant biasing takes care the tubes stays at the same temperature. When it is not biased properly, it will change a bit all the time and never will go in steady state and keep a bit fuzzy and unclear. Especially class A amplifiers suffer from this problem.
• Have a very high load at ac (music signal). When looking at the small signal replacement scheme of a tube, all distortion factors reduce to zero when the current swing and voltage swing are zero (of course, the tube does not do anything then), the current is the main factor. It has to do a lot with the gm changing with current, resulting in production of harmonics. When looking to the curves of tubes (Ia vs Va and gm vs Ia) this can be seen clearly. So tubes have the very nice feature that they are very lineair when the current swing is low. Tubes are very good voltage amplifiers. When a tube is loaded with a very high load, it always is very lineair. Please take a look at these graphs of the ECC83 as proof (of course it is also possible biasing a tube in high voltages and currents, the results will get even better).

So we have to take care the tubes has very solid bias en is loaded with a very high ac load. High load means that there is a lot of voltage swing, but almost no current swing (ie Ia=Va/Rload). This result in very lineair undistorted sound.

Tubes can be loaded in different ways:

• A high value resistor:
  + Simple and cheap.
  + Very high bandwidth.
  - Large voltage drop, and thus very high B+.
  - High dissipation.
  - Resistors are dissipative elements, and are always damping, resulting in less dynamic sound than the other options.
• Active loading like SRPP, or mu-stage:
  + High ac load, because of active loading of the signal tube.
  + Low output impedance, making it possible to drive long cables.
  + Solid dc biasing and good power supply rejection (only when penthodes or Fets are used. Triodes give bad power supply rejection, and are a bad choice for the upper tube).
  - Extra tube and a lot of parts in the signal.
  - 100% current feedback, the sound gets imo very technical and never gets really to life.
• Choke or transformer load:
  + low dc resistance, so voltage drop is very low.
  + High ac load at higher frequencies.
  + Reactive element giving very good dynamics.
  - Expensive and hard to make the right way.
  - Low bandwidth.
  - At low frequencies the load drops rapidly. For example the load at the -3dB point the load is the same as the plate impedance, below the -3dB point even lower. So just at the place where high load is needed most (large low frequency signals) the load is lowest.
  - Bad power supply rejection, because of the relatively low impedance at 50 to 120 Hz.
• A CCS (constant current source) load:
  + Cheap and simple.
  + Very high constant load, in the order of 2MOhm.
  + Voltage drop of the maximum signal only, leaving a lot of voltage for proper high biasing of the tubes.
  + Very high bandwidth, at the low and high frequency side.
  + Very high power supply rejection, about 60 dB!. This supply rejection works also over high bandwidth, resulting in almost complete rejection of power supply hum, but also Hf noide from the socket and switching of diodes, etc.
  + Because the load is a current source, the tube really gets biased solid. At a given constant current and biasing resistor it only can set at one, and one only, anode voltage.
  - I Can't really think of any. Maybe the use of solid state, but we did not hear harshness or anything. In fact, the sound really gets open, dead quiet en very good.

One last note: When using CCS (but also resistor or choke loading) in a driver stage, it is very important to choose the right tubes. A lot of amplification is needed, but also driving current (driving te input capacitance of the power tube) and low output impedance. Mu-stage or SRPP can be used, but this is mostly to correct tube weakness. It is needed when standard tubes like the ECC83, 6SL7, Ba, 6SN7, WE310A, etc tubes are used. All have low gm, meaning low driving power and high output impedance. There are a lot of tubes much more capable for driving, combining high amplification, high current and low output impedance. Examples are 5842 or WE437A, EC8010 or EC8020, but also much cheaper tubes like EC91, E180F, D3a, E810F in triode, EC86, PC900, 6C45pi, etc. All these tubes are very lineair and are great drivers and can be used directly with only one stage on the power tube.