[MC5C]

So I just tried this on a flat-top 17" jumbo I happen to have out. It had clarity on most notes above the 12th fret, except for high G and F# on the low E string. They were dead and thuddy. One thing I do when looking at a dead note is sing that pitch into the soundhole of the instrument, if it resonates strongly back at me that tells me it's just the body resonance robbing the note of power, and there is little that can be done with it. But all the other notes were clear up to the end of the fretboard. So I would say look at the body resonance and see what it tells you. But - I suspect the main problem is lack of tension in the strings so no power at all with such a small mass of string available to vibrate. Your string tension is likely around 15 lbs per string, tuned that far down with such a light set. That's about half normal tension, so the instrument just doesn't stand a chance of working properly