Tamp harder? Dose higher?
The Breville 450 is renown for not grinding fine enough so better off using the Smart for your experiment me thinks (not heard this complaint leveled at the Smart, any owners want to chime in here?). Can you quantify the finest of the grinds from the Smart, compare it to something, post a photo, or get out a micrometer and measure the grinds ;) ?
If youre pretty sure the tamp pressure is truly hard that just leaves the dose (try filling the 2 shot basket right to the top, banging it a bit to settle the grinds and then topping it up a bit more, then tamp really hard). If that doesnt work its a bit of a mystery, blame the beans maybe?