So any time I build a preset with parallel routing, I have to retweak gain in amp block ? Or what is the best approach to this ?
As others have advised, if the drive block is not being used in a certain scene, have the bypass state set to Mute to avoid the gain buildup oversaturating the amp.
If you are using parallel drive block gain for scenes, the key is set up the block output levels when all the drives are on and tweak your mix of drives to taste.
If you need more volume control variation of the drives before the amp block, then put a neutrally set filter block after each drive block. With this method, each block has unity gain plus 3 other channels each that may be used for output gain control with minimal memory cost.
Interested to know if there is something I'm missing as far as how having a parallel shunt could be useful
Parallel drive blocks into a single amp can sound massive. You can bring definition into a fuzz sound using a parallel drive block using the FET preamp set with minimal drive and some righteously set tone controls, for example. A Tube Screamer and a Rat set with complimentary tones on parallel signal paths can sound more complex than just one drive pounding the amp alone, or setting up one drive in series into another drive.