It'll depend on how simple you want the harmonies to be. The included H910/949 will do 5ths & octaves nicely – up or down. So will Crystals (but it's a reverse delay). They both do static, parallel intervals.
If you want intelligent harmonies … 3rds, 4ths, 6ths based on a scale & key … you'll need one of the diatonic algorithms. Diatonic can produce two harmonized intervals locked to selectable keys and scales. It also incorporates delay with feedback. QuadraVox produces 4 scale-constrained harmonies, with a little different method of delay time spacing, and no feedback.
You'll get a free algorithm coupon, so I'd recommend using that on Diatonic for classic, two-guitar harmony lines. And since you're a bass player, check out the system settings. Under Utility, the default SOURCE is guitar, but it can be switched to Bass, SynLead, and SynBass.
