Hi Harriet .. I had one of those Harmonys, too. It's unusual since it's actually a flat top guitar with f-holes like an arch top, correct? It's a small, concert-size guitar, much like the small Stellas folks talk about here. Parlor guitars (lightly made, gut string instruments used in the 'parlor') were long gone by the time these guitars were made.
Regarding your question, I'm thinking you're trying to get a 'baritone' sound and wondering which guitars are suited for it? Your Harmony is like a short-scale guitar, measuring 25" and change. Tuning these guitars down won't allow the tension necessary to produce a decent sound, although heavier strings will help.
Most players seek out a longer scale instrument, like a grand concert-size Stella (Oscar Schmidt-made), which measures 14 3/4" across the lower bout, or an auditorium-size Stella, spanning over 15" across the widest part of the bout. Both of these guitars have the 26 1/2" scale length and can be tuned down easily to A or B. In fact, these are the size guitars that McTell and Leadbelly used, and they are always tuned down, sometimes waaay down.
So the most important thing to get the baritone sound is the longer scale length (measure from nut to saddle). All that said, the larger body Stellas et al are usually a tad more costly than their concert-size cousins, because they are much more rare. You can look on my website (linked below I think) for some examples, or search the web for 'long scale guitar' to see what is out there. I personally love the deep tone produced by the long scale and deep body of these instruments, and am alway on the hunt for them, so I think your question is right in my wheel house, as they say.
I just did a demo of a grand concert Stella-type guitar made by Michael Iucci in the 1920s .. attached to this post, which will give you an idea of a GC tuned down one whole step.
Hope this helps some..Tom
