Christophe, Sten,
Thank you for your kind comments. Really, there is much more to say, and on re-reading, my original post could be read as being unduly critical of the alto. This was not my intention, but just to point out, to use an English phrase, "there is more than one way to skin a cat".
I did indicate that the problem with a 10s in E is dealing with flat keys. As an illustration, I recently investigated Partita Nr. 4 by Kuhnel for 11c lute that can be found in Jean-Daniel Forget's site, with which we are all familiar. The suite is in F, i.e., only one flat, so, one would hope it would be suitable for a 10s in E, perhaps even a standard 8s in E with basses at D and C. I started by considering standard romantic tuning, i.e., basses at D, C, B', A', even though the last two strings are not necessary. This approach was undermined at the opening of the Prelude. There are a series of chords with F as a fundamental. These are difficult/impossible to play with having to fret the F at the third fret on 7, while fretting the treble at the fifth/sixth fret on strings 1, 2, 3. I therefore decided on the alternative tuning, D, C, G, F, i.e., G and F are re-entrant. This worked for the Prelude, and in the following sections, because you have the same basses as a lutenist had, in total, G, F, E, D, C. even so, there are a few chords that do not fit easily or at all on a standard guitar. If one is prepared to compromise on this issue, the main problem with this tuning is that, either, you have to change strings on your 10s, or have two 10s's, if you want another tuning as well ... . I therefore tried the transposing key of D for performance on an alto, and it worked beautifully! Everything falls well under the left hand. In a number of cases, the version for alto made some chords playable that are unplayable, or only playable with difficulty on a 65 scale instrument. So, the alto proved much the more appropriate instrument for this suite, even though we are dealing with only one flat. Playing in D does mean sharpening the 8th, but I view this as a small price to play for ease of performance.
With regard to the tunings used by Yepes and Schmidt. Schmidt uses Yepes tuning for all modern works written for this tuning, and in a number of other instances, but uses markedly different tunings when dealing, for example, with Bach. Glen is knowledgeable on other tunings used by Schmidt. Yepes tuning is standard for strings 1 to 6, but 7 to 10 are as follows: C, A#, G#, F#. These are chosen to support the notes of the treble through acoustic resonance that are not supported through the harmonic overtones of the open strings of the standard 6s guitar. C does, of course, offer the possibility of providing a bass line unavailable on a 6s. The magnitude of the resonance effect is variable with guitar construction, but usually people who try it like the balanced sound it gives to the instrument.
In using Yepes tuning for baroque lute pieces, two/three approaches are possible.
1) For 11c pieces, one can tune down a semitone strings 9 and 10, where low G and F are required, or leave F# for keys G/Em and D/Bm, and G# as well for A/F#m.
Alternatively, one can transpose the key up a tone, e.g., change C to D, and tune up the 7th to D, noting an Aranjuez 7th will do this without difficulty. With this retuning, note that you have E and D as open strings, F# and G are available from 9 and 10, and A is your 5th. Thus, in total you have available as open strings the same bass sequence as a lutenist, though a tone higher.
2) For 13c pieces, particularly where the remarkable Aranjuez 7th string is used, one can tune down the 7th to A' and use strings 8, 9, 10 as in 1). This approach does depend on the feasibilty of being able to fret the bass line as well as the treble, but works in a suprising number of cases.
Alternatively, one can drop the 7th to B', say, and transpose the key. A typical case is changing key from Dm to Em, which would move the bass A' to B'. Here, strings 8, 9, and 10 are used as in 1). Again, the feasibility of fretting the bass and treble may be an issue.
Yepes tuning may involve a player in changes of key, which may be unacceptable to nsome people, and does imply a likely degree of retuning for baroque pieces, but using an alto does not free one from having to retune. In my view, I prefer to say close to the original key, a semitone off, perhaps a tone at most. Where Yepes tuning is unsatisfactory is in renaissance pieces. Setting aside for the moment the question of renaissance pitch, renaissance lute music, including that for 10c, attiorbato and archlute is for an instrument in G. One can put a capo on a 65 tenor in E, but this limits the number of frets, and does not give the same more lute-like sound of an alto. Even if one is prepared to play these pieces a minor third lower, the lack of bass strings turns out to be more of a problem than with baroque music. I have found that for this type of repertoire, Yepes tuning makes the pieces unnecessarily more difficult, and often impossible.
Another long post ... .
James