MIMF

All of the few lap-slide guitars I have or have played have the same problem. The low strings, when barred up to around the 10th fret position or higher, have a dull, thudding sound. This includes two resophonics, a Weissenborn, a vintage Oahu, an ovation fretted guitar with nut raiser, and a few other people's instruments I have tried out. One other Dobro player I have talked to has said they are all that way. I am not sure whether this is the same thing as very low sustain. The treble strings sound fine, way up the the neck.

Heavier strings help, but only a little. Bar pressure seems to make no difference. Neither a really massive lump of steel (about 1" diameter and 4"long) nor a brass Stevens-style bar with multiple lightening holes drilled in it make much difference either.

Any advice on how to correct this?