In theory, any kind of mute can be made for the tuba. In practice, this is generally not the case (imagine a plunger mute for a tuba). Generally speaking, it is a rather safe assumption that the tubist will have a straight mute (used in the sound clip above). Asking for other kinds of mutes should be based on consultation with the performer.
Removing valves is an extreme technique in that it completely modifies the sound of the instrument by producing the tone through the valve casing, bypassing the remainder of the tubing and the bell entirely. Note that any single valve removed will have this effect and the effect cannot be turned on or off (except, of course, by replacing the valve).
The removal of a slide is similar to the removal of a valve in that it bypasses the remaining tubing and the bell, with the important distinction that the effect is only present when the corresponding valve is pressed. Thus, for instance, the first valve slide can be removed without affecting the sound of the tuba until the first valve is pressed. The removal of multiple slides can produce a simple spatializing effect as the sound will be produced from different points of the instrument. Further, because each valve has tubing of different lengths, certain sounds can be given relative pitch difference when multiple slides are removed. It should be noted that any tone produced through a removed valve will be subject to a marked difference in timbre and volume.
It is of importance to consider that certain slides are relatively easy and quick to remove, particularly the short 2nd slide, while other slides are considerably more cumbersome, e.g. the long 4th slide. If the removal is meant to be subtle or even unnoticeable in performance, it can be prepared in advance or kept limited to a shorter slide. On the other hand, the removal of the 4th slide can serve as a dramatic performative gesture.
Though they are quite rare, there are tubas with removable bells. These were primarily made for recording sessions so that special forward-facing bells could be attached. If one is able to find such an instrument, the bell can be removed entirely, altering pitch and timbre to a rather large degree, in effect making the sound seem unfocused. Almost all sousaphones, of course, are made with removable bells, though tubists should not be expected to own a sousaphone.
The traditional purpose of a water bath is to clean out the inner tubing of the instrument. However, if left inside the tuba during performance, it can alter various parameters significantly, depending on the amount of water. Timbre and articulation, for example, incorporate a high degree of indeterminacy. Sounding dexterity is (severely) limited. However, with the tuba’s massive resonance, the sound of water sloshing around can be marvelous, and unlike anything the tuba could otherwise produce.
In playing the tuba, there is a natural buildup of saliva in the instrument. As the amount increases, so do the frequency and perceptibility of the sounds it makes, which are often of a percussive nature, linked to sudden excitations of the instrumental body. This can provide similar opportunities for experimentation as the water bath, or just signal that the player should empty the spit valve!