The HD won't sound weak if split (it's actually a bit fat for a coil split), though I can't say anything about parallel because I haven't tried that setting. If you want series-parallel and a coil split, you'll need to instal a switch, otherwise you'll only be able to split (if I'm not mistaken) the diver. I'd keep the volume pot at 500k, because the Holydiver has more focus on mids and low end (read: it's not too bright). Actually, If I found the HD too bright, I'd actually put in a 500k (or 1 meg) tone pot with a low-value capacitor.