To achieve the best sounding guitar ensemble, consider adding a requinto and/or contrabass.  As nylon-stringed instruments, their unique timbre blends beautifully with standard classic guitars.   The result is a full and satisfying spectrum of sound.  Both instruments are transposing

Alhambra 5P Requinto–open strings

The Requinto tuning is A-D-G-c-e-a, with each string sounding a 4th higher than standard.  Recommended requintos:

Tip: If you can’t purchase a requinto, you can make one by re-stringing a standard guitar.  First remove the 6th string E; then shift the other strings over one notch to the left on the nut.  Remember to tune the B sting up to C.  For the high A string, use fishing line (sounds weird, but it works!).

The Contrabass are available as 4- or 6- string instruments and are tuned the same as standard, but with each string sounding an octave lower.   Four-string contras are tuned the same as an electric bass or upright double bass (which could be used as substitues if you can’t afford to buy). Recommended Contrabass guitars:

Contrabass strings  (Hannabach 841MT–4 string set) or  (Hannabach 841MT–6 string set) are available from Strings by Mail or Just Strings

Esteve PS75 4-string Contrabass open strings