Ok final post of this monologue
Took delivery of the tumba from the tycoon platinum range - a 12.5" drum with a good quality rawhide head. I tried a variety of tunings with four drums - CGDC, CDED, CBbGF as well as CAbEC - but wasn't that impressed with any of them - mainly that there was not enough difference in pitch between the bottom two drums. So decided to keep the tumba at a D, tune the 11.75" conga to G3 and tune the 11" quinto up to C4 - which means I will definitely have to tune it up and down each time I play
- however this makes the requinto redundant because I can't get it to tune up to E4/F4 and having it at D4 was too close to the quinto at C. So I will probably stick with three drums and the CGD tuning but each one a size bigger than before which gives me more volume and depth. I may use the requinto as a solo/portable single drum tuned to C when I am travelling or pushed for space. One day I will save up and buy a decent set of congas with good heads and the tuning shouldn't be that difficult to achieve...
Edit: Tried tuning the requinto down to Bb and it sounded very like a large hand carved African drum I picked up whilst working in Zambia - so now using the CBbGD tuning (high to low) and very pleased with the results - this allows me to play latin and afro flavoured beats - Now I just need to work out the best way to position four drums...