For my twopennyworth, I AM the owner of the 25 year old Sigma that Early was refering to earlier, and having just had it refretted, new bone nut and bridge done by a VERY VERY competent luthier, who looks after some VERY VERY top class instruments, the result is that a) he wants mine, and has wanted an EARLY Sigma for a few years, b) he has copied the dimensions exactly to reproduce one, and c) we find out that it has a centimeter and a half LONGER neck than the Martin!!
This apparently accounts for the huge volume from the beast, but he also agrees that there's bugger all difference in the sound of mine to a Martin. As a matter of fact, I use 11 gauge martin sp phospher strings on mine, other wise if I used any heavier gauge, I woudn't be able to hear myself singing!!
I have so far over 25yrs resisted the temptation to buy a Martin!!!
cheers
Spike