Transformers, either interstage or output are of utmost importance in tube amplifiers (and preamplifiers!) and their performances have a major impact on the overall sonic quality (and stability if feedback is used) of a realization using them. Altough generally disliked by amateurs (and often by professionals too), their have no equivalents in power matching or for driving a stage working in class A2 with substantial grid current (that is the case of power triodes like 845, even when biased in A1 class). Subjectively, most transformers coupled stages are superior to their resistive counterpart: recent realizations using inductive coupling between a driver stage and a power triode output stage (transmitting tube) have proved the interest of that technic. On the other hand, monotriode amplifiers are requiring output transformers with higher input inmpedance than usual and that must withstand a continuous current without alterations of their characteristics. Those points clearly show how important it is for an audiophile to well inderstand basic principles of transformers.
All that will be said in the following can be used designing power supply transformers, altough those are no likely to make hard design constraints arise.