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I was just pondering on building my first harp guitar, and I'd really like the sub-bass strings to be more "smooth" sounding, like an electric bass guitar, rather than harsh and metallic like acoustic bass guitars tend to be. But are there any specific ways to push it toward that direction, or is it one of those "too many factors" questions?

Here are some examples of what I mean...
http://deku.rydia.net/guitar/MetallicBass.mp3
http://deku.rydia.net/guitar/SmoothBass.mp3

The metallic one is a spruce top, mahogany back, 24.75" scale, 14 fret, OM sized dread-style shape.
The smooth one is an Indian rosewood top, bubinga back, 25.5" scale, 14 fret GA.

But I can't find any single factor in either of them that I don't have a counter-example to. Both of those have very thin tops, but are braced pretty differently. The rosewood topper (my experimental guitar) originally had a spruce top that was thicker, and sounded more smooth. so apparently it's not the species or the thickness of the plate. Both have rosewood bridges with bone saddles (although Madagascar with 1/8" saddle on the metallic, and Indian with 3/16" saddle on the smooth), so I don't think it's that. I have a full size spruce dread that sounds much like the metallic one on the low strings, so I don't think it's body size.

Perhaps it's the mass? I don't have good records on some of my soundboard weights, but the metallic one should be quite a bit lighter than either the spruce or rosewood tops I've done on the retopper. But the full size spruce dread should be relatively heavy too, and is more metallic... but it's stiffer too, so maybe it's high mass plus low stiffness = less treble = less metallic?

Does anyone even attempt to control such things, or just shoot for a balanced middle ground in all aspects of the build, and take what you get?

Personally I like the one you call metallic best, nice crisp overtones.

I think you will find that string choice and age will be the best way of achieving what you want.
Put a set of uncoated strings on the subbass and don't change them unless they break.

Rub your greasy hands on them while eating fried chicken etc

The d'addario double wound low B on my baritone is pretty fundamental dominant too.

I'm not sure that "voicing" is where I'd be looking for what you describe but rather the length. tension and type of string for the sub-bass. For more "smooth" sound I'd maybe try classical guitar or harp strings on the sub-basses rather than phosphor-bronze ones. You need to choose the back and side woods as you would for making a baritone to keep string to string clarity in the lower notes - a reverb rich Rosewood probably wouldn't be your best choice.

If you are making your harp guitar with a hollow arm then this will have a big impact on the sound of the sub-bass (and the other strings) compared with a harp guitar with a "broomstick" for the sub bass strings. Also if you are planning to play amplified as opposed to purely acoustically the pickup system you chose will probably have the biggest impact on the sub-bass sound.

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Take your bass strings and lightly run them over a bar of soap.

That gets them sounded like dead strings immediately.

Can you find a cat gut string for a guitar like is used for an acoustic bass? That would remove the metallic sound (which I believe you are correct that the metallic sound is from the treble overtones).

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Formerly known as Adaboy.......

Well I'll be darned, y'all are right. It's entirely the strings, and nothing to do with the guitars themselves Guess that says something about how often I get around to changing them. I thought I remembered the rosewood one sounding like that from the get go, and its string actually looks cleaner (I think it's coated), but it turns out it has divots all along the underside of it from the frets. II just tried swapping the low strings on the two guitars, and here's the result:

http://deku.rydia.net/guitar/SpruceBass.mp3
http://deku.rydia.net/guitar/RosewoodBass.mp3

So that's what I'll do. Rub my grungy fingers all over the sub basses before playing every day, and never change them out. Sometimes things are simpler than they seem.

Thanks guys!

After listening to the sound samples, I agree that the result you desire could be achieved by simply swapping strings. You may also want to try some coated and/or flatwound strings. The only real difference is that one sound has more high frequency harmonics than the other. Sustain is about the same.

That said, in years of building experimental instruments, I've learned there are things you can do. I made a giant multistringed thing with an integrated baritone lap steel a few years back. It had bass that sounded very much like an electric. It had a mahogany top.

If it were my project, I would split the difference and build all mahogany, with tapered bracing and a fairly heavy ebony bridge. Mass will help. I think of it as analogous to inductance, in other words it's a low pass filter, and will EQ the sound towards the bass. I would also suggest thick sides and a moderately loose top. Definitely go for the hollow arm; you need as much air volume as you can get. Aim for a moderately low main helmholtz resonance - maybe 65 or 70hz. If anything, the soundhole(s) should be on the small side. This will lower the frequency as well as the Q of the resonance (making it lower and broader).