Octave stringing
The story of octave stringing on the lute is a long and complicated one. It's not just a question of string technology, though presumably the practice did originate in the need for reinforcement of the bass sound.
For much of the 20th century revival of the lute it was the practice to use unison strings throughout. I think this was because for a long time practically the only facsimile of lute music that was available was Robert Dowland's Varietie of Lute Lessons (1610) in which John Dowland wrote that to use octaves had been common practice "but amongst learned Musitions that custome is left, as irregular to the rules of Musicke". It may also have been influenced by the fact that lutenists had all been guitarists and octaves seemed strange to them. Many people still seem to believe that Dowland advocated unison stringing on all of the bass courses (he was talking about a nine-course lute), but a careful reading of what he wrote does not bear this out. The complete text is as follows:
"Secondly, for on your Bases, in that place which you call the sixt string, or r ut, these Bases must be both of one bignes, yet it hath been a generall custome (although not so much used any where as here in England) to set a small and a great string together, but amongst learned Musitions that custome is left, as irregular to the rules of Musicke."
So the issue for Dowland was whether or not to use an octave on the sixth course. He says nothing about the remaining basses, and all the other evidence we have from paintings and other sources suggests that these courses would always have been tuned in octaves. We have tuning instructions for the 11-course lute and numerous paintings which show that for the "D minor" tuning octaves were used from the sixth course downwards. From a string technology point of view there is less reason to use an octave on the sixth in this tuning as the sixth course on an 11-course lute (new tuning) would be thinner than a sixth course on a 10-course lute (old tuning) and have less need of reinforcement by the upper octave. My conclusion is that it would have been usual to have octaves from the sixth or seventh course downwards on the 10-course lute.
But you could have octaves higher up - there is a beautiful painting in the National Gallery, London, by Terbruggen (dated 1624), which shows a 10-course lute with octaves from the fourth course downwards.
How prominent the octaves are when played depends on many things, not least the skill of the player. At first sight, the usual tuning of the 6-course lute with octaves on fourth to sixth courses seems to go against the contrapuntal style of much of the music, but a skilled player can choose to bring out an octave here or minimize its effect there, as the music requires. Many players today feel that the octaves are an essential part of the sound of this music, more a matter of colour than notes.